i'm trying not to expose people using genAI to harm, and don't want to put a guide on how to avoid being detected out there, so i can only answer this vaguely ;-; also note that this is the current state of things, generative AI seems to still be steadily improving.

in general: everyone has stylistic tells that range from choice of words, sentence and work structure, grammar, themes and characterisation to punctuation and spelling.
generative ai, at least if you use one of the publicly available models not further trained on a specific data set, also has a "style". once you learned to recognise it, it feels like one single person was posting under dozens of different accounts, across all fandoms. and it's a "style" that's present outside of fic as well, a lot of magazines etc. use it too. once you're aware what it reads like, you'll see it everywhere; i can e.g. tell at a glance when people put their handwritten fic into an llm to generate a fic summary, sometimes a title alone is enough.

for everything listed below there will be cases where it's simply a part of someone's style, or they're genuine mistakes we make as writers. it's the accumulation of many or all of these aspects that point to a text being ai-generated. the more human input (editing, expansion) there is, the blurrier becomes the overall impression of a text—but it must be said that genAI leaves its traces (or shall i say: an indelible mark 🥴).

its most distinguishing feature is that it is bafflingly repetitive, to a degree no human writer is. you'll find words or phrases not specific to the text used up to a hundred times in a 100k text, as is not the case for a human writer. and this is across all ai-generated texts, to a point where seeing several of these word or phrase alone can be a sign. and it's not just words—within a text, you were until recently likely to find sentences or entire paragraphs that are a rephrased repeat of a previous one. writers do make these mistakes occasionally, but not as frequently and reliably as genAI, and most importantly: not at the level of writing this "style" would imply someone to have achieved (!), were they human.
they improved retainment of input (and possibly output too) recently, so the repetition of entire paragraphs and sentences was vastly reduced, but can still be present. the vocabulary doesn't seem to have changed, so word repetitions are still as abundant as ever.

a general marker of this "style" is that it depends on fancy words. it's a rather elegant and polished-seeming "style"—on the surface. when reading, it feels like biting into a cardboard replica of a slice of cake. big declarations with nothing behind them.

up until recently, this "style" aside, another big tell was inconsistency. if rooted in real life there'll be fake facts. then there's basic things like a character's age changing several times, time that passed not adding up, seasons not lining up, settings changing mid-scene. the rules of the world were laid out but not followed. the premise of the text got entirely lost to the point where an integral conflict stopped existing or warped into something else. things are said or happen but don't soak into the fabric of the text, characters seem suspended in their initial state.
still true currently: emotions make no sense. llms struggle greatly with portraying conflict and morally grey* areas, so emotional responses do not correspond to what happened in the text prior. usually, this manifests in an "offence" that is barely more than a flippant remark but is treated like the end of the world. conflicts are also often solved, then out of the blue reset to unresolved.
(*note that genAI can be used to generate text of any genre, including omegaverse and mafia aus, and also smut.)

with people who published work prior to the availability of llms, there's a vast difference in all elements of their style as laid out above that is so vast it cannot be attributed to finding a beta reader or an evolving style. most people's writing styles evolve very slowly, over the course of many years. oftentimes, this means the difference becoming visible not just in comparison to older work but within a text itself, when someone generated text and chose to edit or blend original writing into it.

although these texts tend to come with dialogue now (for a while they didn't, or it was clearly written by a person), the dialogue itself feels empty, often reads like therapist speak, and tends to state the obvious. it often doesn't contribute to the plot progressing. colloquialisms are often absent (unless edited in by hand).

another tell: ai-generated text comes without typos. (this stops being the case when the poster edited it.)

to be frank, personally i feel like characterisation is near absent. llms do the same as ai-generated images do: they have no knowledge of the subject so try to appeal to your feelings first and foremost, through heightened emotions, tragic situations, highlighted suffering, and prompt you to empathise so much you won't notice it's not real. if you look closely they're just cardboard cutouts of a character who could be anyone. which is why it's so tricky with fanfiction! we're used to projecting on fic. a fanfic writer can rely on the reader bringing their own ideas about the characters to the table and doing half of the work in this regard. ai-generated text passed off as fanfiction takes full advantage of this.
what is usually absent in addition to this are easter eggs, references to canon occurences like pairing moments, things they said or did. (don't use this particular observation about characterisation in isolation as a diagnostic tool though, many writers like to detach their characterisations from their however perceived personas.)

this comes almost last in this list although one would think it was obvious—but surprisingly enough, plentiful posting does not mean it's ai-generated text. one would think it would! and it can be, but in the majority of cases people post as irregularly or intensely as any fic writer, even leave chaptered texts unfinished. the exception would be those chaptered texts that rack up one million words over the course of three months.

in rare cases i found remnants of the llm responding to the prompt. there's no use looking out for this though, i've come across it maybe twice.

in general, one thing seems true for generative ai users: they generate to avoid the effort of writing and value ideas above their execution, and this mindset affects the quality of their ai-generated texts. although a lot of these texts could be easily edited and fixed—at least regarding most of the issues addressed above, not the general vibe of genAI—the majority don't fix them. make of that what you will.
we all make mistakes as writers, consciously or not, but the mistakes generative AI makes (at least regarding text) are predictable, repetitive, and lack the personal circumstance causing them to have indeed a very machine-y feeling to them.

tl;dr i find it has a noticeable "style" and also find it most bewildering to read, like setting foot into a palace with a perfect exterior that looks like a site of demolition on the inside. inescapably coming across it forced me to grow attuned to its telltale signs :( i'm not alone in this, many writers, publishers and teachers find it to be very distinct and obvious, partly due to lack of individual style, partly due to mentioned carelessness. i like to think it's a skill that can be learned through exposure (and also reading carefully and critically), but understand why people would choose not to spend leisure time reading ai-generated text, and also choose not to scrutinise each fic they come across. might as well be that you have nothing to worry about, or at the very least not yet. good luck out there ;-;

thank you for asking ;u; the exos on the just some guy spectrum:

just some guy-est of the bunch: kyungsoo—but i think it's partly on purpose while also for both impostor-ish syndrome and privacy reasons (see him regularly denying that he sings, acts, cooks etc. despite doing it all extremely well). he is an actor after all, you cannot trust them! (evidence: kyungsoo having the just some guy vibes down to perfection.) it's similar for jongdae! though i think it's more of a carefully constructed and meticulously upheld exterior for him (he is such a private person!) than for kyungsoo whose vibes are half-intended, half by accident. the funny thing about jongdae is that he might very much be just some guy beneath that exterior (idk if that makes sense but... yeah).

yearns to be and/or thinks of himself as just some guy (in a negative way) but hindered by his inherent cuteness: minseok. he's both very self-assured and to a degree self-sufficient (see him often enjoying himself in the company of the exos while literally sitting at the side, it feels like he's often in conversation with himself? remember all those ladder scenes of him making noises while eating, they're not meant for anyone else! it's a monologue, he enjoys the role of a more or less distant spectator. i'm so bad at explaining this, forgive me) but often seems to seek approval to an extent that negates this? okay i feel like i lost the thread here, i cannot explain it! but minseok is decidedly Not just some guy.

look like just some guy but the second they move/emote/speak it all falls apart: sehun (he's so weird, it's wonderful), yixing (strives too hard for the exceptional and undoable, plus also very weird).

would be considered and perfect at being just some guy if he never left his house or showed up to social functions: baekhyun.

isn't just some guy and very upfront about it: jongin (once you get past the shyness, he's this strange mix of incredibly silly and just as incredibly mature, and his matureness makes him exempt from being just some guy in my opinion).

too tall (and impulsive) to ever be considered just some guy: chanyeol.

the opposite of kyungsoo (and to some extent similar to minseok), as in, could never be just some guy even if he tried: junmyeon.

🙈

hi! this is such an interesting and difficult topic. as a reader, i think it depends a lot on your ability/willingness to ignore certain things? we probably all have certain pet peeves, depending on what we're knowledgable about and familiar with! and of course the delivery of a story matters, as do the expectations we have for them. someone writing fantasy is different from writing fic set in a realistic world. but even then it's fiction... with fanfic especially i don't expect anyone to do the kind of research someone might do for a traditionally published book!

the question probably is: does that truly take away from your enjoyment of the fic? you have a lot more power as a reader to ignore certain things than you might think! and still it makes me anxious when e.g. someone writes about a characer being bitten by a cat and it's treated as a regular non-threatening injury that doesn't require medical attention. (cat bites are dangerous, please see a doctor if you can.)
then--are they perpetuating actual harm? also depends on how popular their fic is.
in how far unconsciously do and consciously should we let fiction inform our opinions and actions? this is a highly debated topic, and i think you should find your own answer to this. the one thing i find worrisome at times is that fanfic is most often inspired by other fanfic... it's a weird circle, and we repeat things we have seen in others' writing to a great extent. sometimes it's a matter of wanting/needing to fit in and it being how things are done. think about a lot of tropes playing out a certain way in the majority of works. think about smut--there definitely is one "official", or maybe more like... traditional way to go about it, like it's some kind of secret code or language you are supposed to pick up on and that readers expect you to speak. and in this case it again is mostly a matter of--do you have expertise in this? do you see your experiences reflected? is it fun anyway, and what do you want from smut, and in how far do you want it to deviate from (your bodily, experienced) reality? and not to mention the writer's intent. and all these can coexist with the fact that most people do not bother to learn where prostates are located and that they're in finger's reach (and even without penetration!). is this annoying? oh yeah! does it do any harm? not so sure tbh lmao but probably also yeah. would it hurt writers or their fics to not write prostates as if they're deep inside someone's body? absolutely not. but again, what you make of this is your decision! what worries me personally more than people getting stuff wrong is--when someone "violates" this traditional "code" by adhering to biological realities that also align with their own experiences... and then getting harassed over it--for writing it the "wrong" way.
i picked this example because it also is something that few people receive adequate education on--for people who (want to) have certain kinds of sex, fanfic might actually be the one source, next to porn. idk about you but my sex ed was firmly concentrating on basically pregnancy 101 for cishet m/f couples (and even that's a Lot of sex ed for some people). it's of course not a singular fanfic writer's responsibility to provide sex ed for anyone! my issue lies more with the fanfic smut culture itself i guess that is unquestioningly reproduced--and does sometimes also feed into (cis)heterosexist stereotypes about men who have sex with men (e.g. the absurd focus on penetrative anal sex). i am a gay man with a fundamentally different lived reality from this, so this is something that--while i don't see it as overly harmful in the grand scheme of things--does annoy me and distress me at times, mostly because nearly all smut shows me that there's no room for gay men like me? as you can see this dips into the lack of having one's existence acknowledged, so somewhat existential questions now. it's a very complicated topic for sure!! so yeah sure i sometimes wish people who dedicate a lot of their time to writing smut would at least type some things into google once and look at the first two or three reddit threads on it, if they don't want to waste a lot of time.

all that said--i'm probably on the same page as you. if i bring up medical conditions, i will at least do the minimum research (skim one or two articles from professional resources) unless it is a lived reality of mine (and even then i might do it). i wouldn't think of this as a reflection of someone's skills or dedication or anything--and i wouldn't judge anyone for skipping this, time is precious, fanfic is for fun--but more of my personal level of comfort to make sure i don't have glaringly huge issues in my fic. i'm probably more on the overly cautious side of things here! i once looked up statistics for the most sold cooking oils in sk for a throwaway mention, and i'm like navel-deep into researching seals for a selkie (= fantasy!!) fic. i think being both a slice of life writer and someone who recognises that it is fiction but also can't and doesn't want to separate their politics from their writing informs how i feel about research. i do feel a sense of responsibility to do my best not to write things that could cause harm or perpetrate certain -isms, and also try to write things as close to life as i can, although no one would come and bug me about the details of my slice of life vs. something with an effect on actual people that i would definitely need and want to un- and relearn.

that said--it sounds like you could use a reminder that it's just fanfic? if it gets to the point where you stop writing because of it, maybe you could just use a simple disclaimer at the start of your fic and address these things. just say you're not a medical professional and ask your readers for their suspension of disbelief! i value that kind of honesty, and i think it helps connect with readers. maybe you could also stick to simpler concepts, a lot of fiction glosses over the details of medical things and focuses on the effects and the emotions--that's usually what we're here for! like, don't overdo the research, it's probably not necessary, especially when you feel so cornered by your lack of accuracy (that very few people might even notice). you're not writing for a medically educated audience! people should not judge your work by that alone! your fanfic is probably about something else, at the core. maybe you're focusing more on achieving perfection (which is... not something you should be striving for, there are so many more things we get from writing fanfic imo and you will always disappoint yourself by not achieving it) than enjoying the writing process? sometimes, i take this as a sign that i should find a different way to achieve whatever i want characters to go through. depending on how you write, this could mean a lot of issues with a plotted out draft of course... but it's still worth considering. another solution would of course be to get a sensitivity reader whose personal experiences could also be worked into your fic, but idek if people really do that with fanfic, and it's a lot to ask from someone without compensating them for their time and labour.

it's really about where you personally draw the line--and whether your readers happen to be the kind of reader who knows more about the topic you're writing about than you do.
as readers, i personally think we should extend as much grace as possible towards writers, always, in most regards (really harmful or violent stereotypes aside), and concentrate on the things we like about a fic or someone's writing as well as keep criticism of this kind out of the comments (and generally everywhere fanfic writers would see it, unless explicitly asked for). and lastly simply move on to other writers if we find ourselves repeatedly uncomfortable with someone's writing!

i am SO sorry this answer turned out to be this long (i should probably pay you for reading this...). hope you found at least one thing to take away from it ;;; and tysm for asking!!

hi! i'm somewhat sure that they're active on other accounts in other fandoms too... but i'm not going to chase them down. ty for this message and your suggestion, i was actually considering it last night! but i think most if not all of these services are geared towards students/academic writing, and the free version shows that it found Something but not what (very much suspect it's the "original" fic). from what i gathered they also have a 150k limit per month which isn't very helpful. and lastly (that's not what you suggested, i know!) i am not paying money for this, on principle, especially not when the chances are so slim any service would actually find something.