People will generally write in order to transfer information; perhaps a story, some information that’s new to you, or to change your mind on something. AI produces text in order to fill space. Empty paragraphs devoid of information but which continue anyway are the give away; the Wiki article has some more telltales. You might have some false-positives on people who write like machines, but you’re not losing anything by ignoring their writing.

AI writing also tends be free from fucks, spelling mistakes and odd grammar, since it’s humans that throw those into their prose.