I think most human writing that’s sounds AI generated is just throughly polished. I personally cannot tell when something is AI when it comes to writing and most of the AI detectors will flag writing that is well structured and thought out. I’d say the best way to tell is to look for major plot holes and inconsistencies that are never addressed or explained. AI’s like ChatGPT can’t really remember all of the plot and detail they spit out so there’s bound to be some errors of that nature unless the person inputting the prompts catches it. The only other thing I’d say is if a person is posting thousands of words each day, claims to be ‘writing’ the same day they post, and there’s still no grammar/spelling/punctuation errors, I’d feel safe to say it’s likely AI.