The final output image is just a grid of pixels, just like any other image. Assuming the image has no metadata or has been stripped of metadata, how do you really tell the difference in the first place?

Sure, you can look for JPEG artifacts and chromatic noise and all, but it's pretty easy to overlay that on top of an AI generated image to make it appear more legitimate at a passing glance.

I really don't know a good answer to your question right now, but I'm definitely interested in whatever useful answers others might offer...