There’s no realistic way to enforce that.
You can still mandate through laws that any AI generated product would have to have a label on it, identifying itself as such. We do the same thing today with other products that are manufactured and sold (recycling icons, etc).
As far as enforcement goes, the public themselves would ultimately (or in addition to) be the enforcers, as the recent British royal family photos scandal suggests.
But ultimately Humanity has to start considering laws that affects the whole species, ones that don't just stop at an individual country border.