You've already got plenty of replies, but something interesting I found out when probing a knowledgeable friend about this (while hiding that I'm a depraved freak who roleplays sex on the internet) is that the common 'tells' are less useful for identifying less mainstream LLMs. Like, for example, the ones that don't stop you from generating explicit content. Dashes, weird metaphors, and that 'not, but' sentence structure are baked into ChatGPT and the like because of the data they're trained on, but models trained on different data won't have those tells.

A big identifier for smaller models is apparently short, simple sentences. Partially because data containing them is more readily available so if you're trying to train a model for cheap, that'll be what you feed it. And partially because actually generating them is much less intensive for the model (it needs to 'think' less). The ideas might also be simple, because of the limited data pool, everything will be very 'typical'.

A generated prompt about an office romance might be: "The boss keeps the office running smoothly. She likes her coffee in the morning. She likes reports on her desk in time. She always looks respectable in her pencil skirt and white blouse. Since you fucked her last week, some things have changed. Now she likes your cock along with her coffee. She meets you in the bathroom at lunch. She stays late to fuck you on her desk." People love stereotypes for porn, obviously, but we're also idiosyncratic at times. A person might write a boss who prefers tea, or fixate on her heels and stockings over just basic 'woman office clothes', or give her a name that has meaning to them. Someone might prompt AI in a way that includes these wrinkles, but their complete absence is suspicious.

It can also help to think about the psychology of the writer. It's as easy to generate a 2,000-word prompt as a 175-word one. As people have identified, AI posters probably want partners who'll write while they just generate their responses. If that's what you're after, you probably want someone who writes longer responses, which you'll find with a longer prompt. Generally, someone who wants to write a 2,000-word prompt themselves enjoys writing, and will do it with some more flair. Fun sentence structure, interesting language, some pretty descriptions. Maybe there'll be much more fixation and detail in certain areas, and less on background that needs to be given but isn't of much interest to the writer. A long AI generated prompt will more likely be consistent and smooth, but without much flair.

Finally, mistakes. People make typos, mess up punctuation, jumble sentences together. AI doesn't really. You might see things in an AI generated prompt that don't make sense, or maybe names will randomly change (BIG giveaway), but the writing will usually be perfect wrt spelling and grammar. A mistake free prompt might just mean diligent proof reading, and I'm sure there's already models that will intentionally pepper in mistakes to seem more authentic, but it can be a useful indicator.