Number one indicator? A single punctuation mark that's trivial to make on most keyboards (option-dash on macOS). And generally people who write software are extra fixated on punctuation for obvious reasons: missing semi-colons break your build, etc. Maybe in some other niche message board people will use dash and em dash interchangeably, but here?

Also, if the a single character is how you're red-flagging LLM output, do you know how easy it is to avoid? I didn't use it here at all, but how do you know I didn't run this through some slop-machine to tighten my prose? It's really low-effort take to say "just avoid em dashes so we know you're not an AI".

https://www.mcsweeneys.net/articles/the-em-dash-responds-to-...

Yes, number one indicator. Yes, of course you can go through the output and take out all of the em-dashes. Then the number one indicator will obviously not work.