The only people that have a prayer at doing this, is OpenAI themselves. It is likely they can insert an undetectable watermark in sufficiently generic text output for sufficiently many words which does not distort the meaning or quality appreciatively.

However, there is almost no way this can survive subsequent finetunings.. Like 'rewrite the previous paragraph with three new random words that doesn't change the meaning', and 'change all the nouns/verbs into synonyms that preserves the meaning of the paragraph'.

I strongly suspect (and might one day try my hand at the math) that there can be no such system that works in general against this sort of attack.