Anecdote: I asked ChatGPT whether it created a ChatGPT created poem (not generated by me) and it answered yes. I asked it about one it didn't create and it answered no. I repeated this several times and it was accurate in all those cases. Of course, it may give the wrong answer any time.

The best solution will be for ChatGPT to have a way to verify that certain creations are not generated. Just like identifying duplicate content using tools like Copyscape. Of course, there are issues like user privacy which will need to be taken into consideration, but that seems to be the only fool-proof way.