By the fact that it was generated using GPT. Same way you would go about classifying something as e.g. not made with slave labour or made with a production proc

By the fact that it was generated using GPT. Same way you would go about classifying something as e.g. not made with slave labour or made with a production process that follows environmental pollution rules. That you can't easily detect it from the end product is not necessarily an obstruction to legislation.