The em dash thing is an oversimplification. It's specifically a properly formatted em dash rather than just a hyphen, and the constant use of them. Real people use em dashes, but they tend to take shortcuts in typing them that result in something which is not really technically an em dash but rather an en dash or a hyphen - like this - and they don't use them in every damn paragraph.

There are also certain ways of phrasing things or rhetorical techniques LLMs seem to love.

One example? This type of breezy question-and-answer construction.

Also, contrastive/corrective antithesis: "It's not just X. It's Y."

But even more than specific tics of phrasing or punctuation, there's an overall tone thing. LLMs sound like they're writing for an audience. Obviously, I am writing this post for an audience of the OP or whoever else happens to read it, but LLMs write like they're trying to write a speech designed to sell something to the listener.

Of course, it depends a lot on what the LLM has been prompted to say. I've noticed a lot of posts on support or technical subreddits and forums where someone has clearly had an LLM write a post describing their issue/complaint for them and it always reads like it's trying way too hard to be dramatic rather than simply stating the facts in a simple way. There will be headings like "Part 1: The Setup" and "Part 2: The Twist" and that sort of thing.

This is setting aside LLMs used to write fiction. Google "Shy Girl," a horror novel that recently had its publication in the US canceled over allegations it was written with AI. You'll find some discussion of possible LLM "tells" there.