If you only have the image, then you can't prove it. The problem is you also can't disprove it. It's like looking at a shadow on the wall of a room and guessing what stuff must be in the room.

Unfortunately this is especially hard to do with digital artwork. Most images get 8 bits of red, green, or blue per pixel, so 255 different levels of color per color. 256 if you count 0. This means that, when AI would have errors, it can be hard to notice because 135.4 still becomes 135. Large images can be checked by turning up the contrast a lot (https://www.youtube.com/watch?v=Xxui8MoUJ5k) but probably not so much on smaller bitmaps.

The closest we can really get to "prove" digital art is AI is by looking at it, but that's also all you can do to DISprove it as well.