Absolutely proper and correct use of em dashes, en dashes, and hyphens is, to me, the most obvious tell of the LLM writer. In fact, I think that you can use it to date internet writing in general. For it seems to me that real em dashes were uncommon pre-2022.
(edit: apparently only on Mac, see reply below)
You just type two hyphens (--) and Word will convert it to an em dash.
Typing <word><hyphenminus><hyphenminus><word><space> yields an em dash.
Typing <word><space><hyphenminus><hyphenminus><space><word><space> yields an en dash.
That this has been true for some 3 or 4 decades makes me doubt all the comments that em dashes are a "tell" of LLM authorship. On the other hand, I guess when we confine this possibility to web content, I can see how people haven't used Office for web authoring lately, and whatever they do use (like web-based content management systems) don't tend to have this feature.