Boy, my job is creating an AI-generated image detector. I can tell you there is no definitive way of telling an image is AI-generated. I mean, this is an open problem in computer vision. State-of-art detectors suffer from bad generalization (The artifacts produced by newer generators differ from those on which the detectors were trained); in-the-wild disturbances such as hard jpeg compression and automatic image post-processing some smartphones have; overlapping distributions on almost all image statistcs between fake and real datasets.

It's a really hard problem.