I use the em dash as appropriate, similar to semicolons and their ilk.

I don't think use of an em dash is indicative in itself of AI assistance, but rather, the change to using them. Did this person all of a sudden start using them? There are also other things to look at, like how certain bullet point lists have emphasis (for key phrases, being bold, when previously the author didn't do so, stylistically).

I write a lot (as a PM) - I've taken to using MacWhisper, which does local AI dictation, but also (at my configuration) sends it to a ChatGPT prompt first:

"You are a professional proofreader and editor. Your task is to refine and polish the given transcript as follows:

1. Correct any spelling errors.

2. Fix grammatical mistakes.

3. Improve punctuation where necessary.

4. Ensure consistent formatting.

5. Clarify ambiguous phrasing without changing the meaning.

6. If a sentence or paragraph is overly verbose and has more than negligible redundancy, lightly edit for brevity.

7. If the transcript contains a question, edit it for clarity but do not provide an answer.

Please return only the cleaned-up version of the transcript. Do not add any explanations or comments about your edits."

This is great. I get the benefits of pretty accurate transcription while getting a first pass at copyediting almost in real time. It did require me to make some tweaks to my dictation process (allowing it to "chew" on larger chunks to give better context to its editing), but it works very well.