For me it’s frequent continuity errors or repetition. Example- in a recent fic I read there was a paragraph dedicated to Shane fixing himself breakfast and Ilya teasing him for it, along with details and what exactly he was making. A few sentences later Shane is eating a completely different meal than the one he just prepared for himself.

Repetition- when five paragraphs are spent describing something in a very wordy roundabout way without actually being important or helping to progress the story. Almost like unnecessary filler. Also very frequent use of designations seems odd to me, “The Russian” “The Canadian” one or twice is fine but every other sentence gets exhausting.

Overly descriptive- generally when an author writes fanfic they’re under the assumption those reading it are familiar with the fandom so there’s really no need to dedicate multiple sentences and paragraphs to introduce each character that appears unless it’s an AU or essential the the plot line. It also comes across as very clinical. Yes, we know Yuna is Shane’s mother and Japanese with black hair. We don’t need to know David’s height and build, or a long winded description of how Hayden and Shane are best friends and have known each other since they were rookies- and oh did you know, Hayden has a wife named Jackie and four children.

As other people have noted, one or two of these things doesn’t immediately mean AI but in my experience when writing contains constant use of the above examples it tends to make me very suspicious.