In some fraction of cases, it's really obvious.

I would argue that those cases are really the ones that cause an LLM-specific harm, i.e., which make people feel like they aren't exclusively among fellow humans.

If someone posts something that doesn't clearly read LLM-ish, but is otherwise terrible, it's not really different from if the same terrible thing had been written by hand.

I don't think anyone who objects to LLM comments is really demanding a super-low false negative rate. Just get rid of the zero-effort stuff. For example, recently I've seen a lot of comments from new accounts that are just sycophantic towards TFA and try to highlight / summarize a specific idea or two, but don't really demonstrate any original thought (just, like, basic reading comprehension and an ability to express agreement). And they'll take a paragraph to do so, where a human with the same level of interest in the material might just say "good post" (granted, there's an argument to be made for excluding that, too).