Having been reading generated comments almost daily for over three years now, I have a pretty good sense of it. There's a bunch of signals: how new the account is; how the comments look visually (the capitalization and layout of the paragraphs, particularly when all of one user's comments are displayed in a list). Em-dashes and short, emphatic sentences, make it more obvious of course.

There are cases that are more borderline; usually when someone has used a translation service or has used an LLM to polish up a comment they wrote themselves. For these ones there's less certainty, and whilst we discourage them, we're not as rigid in our aversion to them or as eager to ban accounts that do it.

But ones that are entirely generated are still pretty easy to spot, even just from visual appearance.