"this fic uses em dashes, so it must be ai-generated" real humans use em dashes.
"this fic has long paragraphs with overly described details and scenes, so it must be ai-generated" real humans can write like this.
"this fic has inconsistencies, so it must be ai-generated" real humans make errors and mistakes. that's why we have this thing called plot holes. sometimes writers are tired and they don't remember what they wrote in the last sentences or paragraphs, let alone chapters.
"this fic sounds robotic and unnatural, so it must be ai-generated" not every writer writes in their native language. sometimes they can sound 'robotic and unnatural' if they wrote in their second or third or fourth language (and kudos to them).
"this fic has a prompt left in it that the author forgot to delete, so it must be ai-generated" the 'prompt' the author accidentally left in their fic could actually be a part of an outline that was meant only for them, so they could keep track of what they would write.
"this author posts too often, no human writes this fast, so they must use ai" 1.) you don't know how fast someone can or can't write, how much time a person has in a day or how motivated/skilled they are. 2.) the frequent updates you see could be something that has already been finished and sitting in the author's drafts for god knows how long. just because it's recently posted doesn't always mean it's recently written.
my point? no, you can never know if a fanfic is 'ai-generated'. unless the author says they use ai, you're just assuming, suspecting and witch hunting. chances are that you're not going to 'stop ai fics from being created', you're just going to wrongly accuse genuine writers of using ai and ruin their day at best, make them want to quit writing or sharing their works at worst.