usually, slang either sounds similar to the words it originated with, or combines the first character of each word, so you can guess them the bigger your vocabulary is! if my vocab isn't good enough or my brain isn't connecting the dots well for me that day (or it's a meme or something), generally a google or naver search depending on how niche i think it is. i try to read more than one thread if i can find them, usually one explaining in english and 1-2 explaining in korean. hinative is a decent starting point for when you can't find something on naver or any korean resources because your search terms are too broad or there's too much aegyo for them to have results, but it's not reliable to get answers from on its own. italki is generally better even though it's also basically crowdsourcing. openslang dictionary is difficult to filter but sometimes has things i can't find other places. koreanary is good to try if naver dictionary doesn't have it. if you search in korean you'll find more than one site that explains slang terms, but just like how urban dictionary has incorrect slang definitions, so too do those sites, so you usually should cross-reference a whole lot!