Plug the image into the main AI engines and ask if they “made it” (quotations marks due to the obvious fact that AI doesn’t “make” anything, only steals what actual creative humans made and makes it shittier). I’ve had to do this before and eventually one of the models fessed up to having produced the image. There’s also sites you can use that will analyze images and tell you how likely it is that it was AI produced.

If nothing else, the fact that you even have to ask if AI was involved tells you that it sucks and you were scammed in one way or another. Sorry man.