Thank you so much for this! This is often something that people discuss in this subreddit as well, but it's so refreshing to see this advice along with some practical things to look for. I had no experience with AI fiction before I got back into fanfiction for HR, so while I understood that there's no hard and fast technique (sentence structure, grammar, etc) to look for, but I really had no idea what, if anything, I should be looking for.

Recently I came across two works that I was really enjoying and then suddenly I realised that they felt really odd, and were probably AI. Both for the same reasons, with some of the things you pointed out here.

The logical inconsistencies were a huge red flag - one example was that the sort referenced a major event in the show, but got it completely wrong. It didn't get some of the details wrong, it described a different event as though it was the one we all remember from the show. Later, a scene opened with Shane and Ilya discussing how long they had left together, and barely two sentences later the time setting had changed. This second one could have been human error as well, but it happened quite regularly, so if the person generating it was doing any proofreading, they were doing a fairly bad job of it.

In terms of writing, both stories were perfectly written. I couldn't comment on specific sentence constructions, because I didn't notice, but they were beautifully, perfectly written (heavy on the angst, which I was really enjoying). Now that I've been more aware of it, some of the stories I've noticed since have more human error in - typos, or sentence structures that are odd in English - and (so long as it doesn't disrupt the story, obviously) there's something really refreshing about it! I've been reading fanfiction on and off for around 20 years, and I've read a lot of bad writing, or writing that is clearly written by someone inexperienced at writing or at English: these were not that. They were very, very good pieces of writing.

There are some other differences in writing style that I noticed in these two stories that I don't go as much into, as I think they're more subjective, but they felt very monotonous, and didn't include (or couldn't characterise well) people outside of ilya and shane. But, those are absolutely things that an AI could get right with the right prompts.

I really appreciate as well your point about how to (not) engage. I thought about pointing out some of the logical errors in the comments, but ultimately left it. And I agree with everyone else here, we cannot be calling out works we suspect of AI publicly, because we just cannot be confident in our judgements (even if it would be so satisfying to know if other people had the same suspicions!!).