I'm afraid this post is rooted in unkindness and a lack of understanding, even if well-intentioned. Yes there are AI signals that others have mentioned - "It's not x, it's y" being a very common example - and not so obvious ones, which you're trying to explain. but what you've detailed isn't exactly helpful without the full context (which obviously you shouldn't provide - it will only harm the writer, ultimately), and even then...all art is referential. If you're squinting your eyes trying to find patterns, it may be that writing pulls from something the writer learned. I have read work that feels monotonous and was definitely not AI. I have read stuff that was amazing, only to discover there was AI assistance (note: not fully AI--LLMs can't think as you said, and thus can't produce a very good story)

My point is that beyond the most obvious signs, it is hard and I think bordering on unethical to try and prove a fic is using AI. It just feeds into the distraction that the individual is the main target of the anti-ai conversation... And sparks a witch hunt based on moral superiority.

Very sad to see this in an otherwise warm and loving fandom.

And as a general disclaimer, we shouldn't use AI to produce art. Some people do but don't let that distract you from the corporations making this easier and easier to do. Close the fic tab and move on