so first off i'm not one of those people who noticed the varying writing styles despite reading most of their works, and also didn't give any thought to--as a friend pointed out--the many fics with very intense themes and high word counts they published within a very short span of time. that's also because i'm a very slow writer and honestly cannot tell what's doable for other writers!

what tipped me off was that i recognised one of the books they plagiarised, i've read it several years ago and found it very strange that they didn't mention the book in the author's notes at all. since this was one of the more heavily edited fics, written from a different pov than the original and i didn't have the book around for comparison, it just left a very bitter taste in my mouth. couldn't let it go though so i tried searching for some of their fics by basically googling keywords? that was not successful at all but i did find the folktale in creature of ash and twilight that was copied off a website with only minor edits and again no mention of the source in the author's note. when vargavinter posted their fic i knew it was theirs from just the pairing/theme (they often had these deeply traumatised main characters...) and i read some of it for curiosity's sake? and honestly i cannot tell you why but i pasted a line of the throwback into google and found that several paragraphs of an atlantic article about war photographers were copied&pasted, again only minorly edited, into the fic. to be honest this was as far as i thought this went! just a bunch of aus without mentioning what inspired them, and a folktale and article pasted into their work. i kept talking to some friends about this though (sorry for annoying you all, and thank you for not telling me to shut up ;;) and then somehow pasted the first few lines of their vargavinter fic into google... and found the website of the translator i linked in the doc.
and that's basically it! i kept pulling up fics and pasting lines into google and for most of their recent ones, i found the actual sources. some were (and are) hard to find due to very heavy editing and rewriting--but it's still plagiarism if you rewrite most of a book just to suit your fanfic needs instead of writing an au inspired by something that also mentions it as the inspiration, like everyone else does :(

really the only tip i have is read books? and listen to your gut feeling! also maybe to not treat fanfics like something that only exists for consumption but honour the work behind them. i now think that it's very strange they never (if i remember correctly) talked about the fic in the author's notes; you'd think someone had things to say when they're repeatedly writing characters working through traumas as deep as the ones in the original works they liked to plagiarise? like my friend pointed out: that takes a lot out of someone to write. there's frankly no need to be afraid of every fanfic being plagiarised since there were so, so many warning signs here! i think the trust we place in fanfic writers is something very precious that should not be harmed by rare occurrences like this one. (also there's trust i'm placing in you right now not to be jmc and ask for a reverse manual on how to plagiarise works so they're undetectable)