Would you say there is any way to definitively prove a text is AI generated?

I believe it's impossible to prove that some text was generated by an LLM simply due to the limitations of the medium. At the end of the day, text is just a sequence of bytes, and a keyboard can create a byte-identical sequence to an LLM generated one.

Of course you can run various statistical tools, judge a text by gut feeling, or look for specific clues that LLMs usually generate more than humans (e.g. em dashes). But none of those will ever be a proof. Just indications, which together could add up to a very high likelihood.

IMO, the only way to prove an LLM generated some text is to see the original chat session.