I've seen a fair number of cases where someone swears up and down not to be using AI to generate responses, but there's no good reason to believe it (except perhaps specifically for the messages where that claim is made).
This includes times that someone basically disappeared from e.g. Stack Overflow at some point before the release of ChatGPT, having written a bunch of posts that barely demonstrate functional literacy or comprehension of English; and then came back afterward posting long messages with impeccable grammar and spelling in textbook "LLM house style".
There are a ton of people like that, but the LLM house style also exists because a ton of people write that way too.
The people falsely accused because they've used em-dashes for 20 years aren't the ones that were functionally illiterate before.
I don't think there's any definitive way to check, but for me one of the biggest tells that a long piece of writing was LLM generated is that it will hardly say anything given how many words are in it.
(well that and the "it's not just x, it's y!" pattern they seem to love)
I think em-dashes were uncommon mainly because they're not always convenient to type.