My gut feeling is that machine learning is pretty good at this kind of text categorization. You could also try going old school and using stylometry. There are

My gut feeling is that machine learning is pretty good at this kind of text categorization. You could also try going old school and using stylometry. There are a bunch of companies trying to sell GPT detection. But also remember captcha stands for Completely Automated Turing Test to Tell Humans and Computers Apart. Automated isn't necessarily always a hard requirement, for a Turing Test just using your gut is fine.