If you turn on HN's "Show Dead" setting, there are tons of LLM-generated comments on stories related to AI. You can see the human(s) behind the LLM trying to fiddle with the style of comment by making them skip proper grammar, capitalization, use or avoid certain phrases, and so on. The biggest tell for LLM content, though, is just the content as a whole: it sounds fake and ungenuine, like it passed through a committee of hostage negotiators to remove the speaker's own attachment/expectations.

They can configure it to use all lowercase letters, skip em-dashes, make grammar mistakes, stop saying "it's not X, it's Y", or whatever, yet the content itself just has a fake quality to it that makes it stand out, which is why those comments still get flagged IMO.