the only time i located ai usage and it was confirmed by the author was when it was an ai translation and i mostly did it by gut feeling. theres some others that i suspect are ai through the same gutfeeling (like too perfect sentences, words/meaning/phrase repetitions, or random double spaces in the middle of the fics), but i'll never know bc theyll never be confirmed. the shit thing about ai usage is that its really hard to spot, and even if you "spot" it, its really easy for the author to refute it even if theyre lying.. and often times they might not have written the whole thing with ai but just used it as an aid.
its not worth losing your braincells and time over "figuring out" if something is ai. id recommend putting that energy into writing your own fics or supporting authors who you know write their own fics. uplifting is powerful!