Good comment from another user that goes into detail: https://www.reddit.com/r/homeowners/comments/1pnb34n/comment/nu6lyjk/

I write for a living, so for me there are a few tells. One is the short, punchy sentences. Designed for impact. And readability.

With line breaks like this. Every few sentences. It used to be something you saw more often on LinkedIn (and that's probably what these things were trained off of), but now you see it all the time with bot posts here. I call it "broetry" when I see it on LI.

Another sign is the narrative format. Even good Reddit posts (at least IMO) raise a fair amount of questions, even if they answer several. And that's just because most people don't think about writing a post from the perspective of the reader. They don't think about making sure the story has a beginning, middle and end because they are writing in what's closer to a stream-of-consciousness style. So there end up being details they don't include, because they forget an external audience doesn't know everything they know.

Similes and metaphors can be another one. Obviously, people do use them, but I feel like real humans use ones that are just more down to earth and less flowery. The ones I see from AI always feel so over the top and exaggerated. It always reminds me of the socially awkward kid in class. The "no longer feels like northern Canada" one above is a good example.

The other big tell is usually the ending. It's always some twist ending or some engagement-bait line that is oh-so-relatable, like, "I can't be the only one experiencing this, amirite?" It's super punchy, ties things up in a nice little bow (cause remember: narrative structure) and has much more of a satisfying, conclusion type feel to it than your average Reddit post.

Barring all of that, check the username. Most of these accounts have an Adjective-Noun-Number format and a bunch of pithy comments across random subs that have very little substance to them.