There is no entirely deterministic yes/no process that can prove that an image is AI-generated from the image itself; it definitely looks AI generated to me from a gestalt impression, but like I said it looks like it was traced over by a human artist rather than being the original generated image, which also would mask any really clear “tell”. The facial expressions are generally a bit incongruous and reminiscent of AI images, the use of color is a little odd, etc.
However what makes me fairly certain is the (in this case, quite strong) circumstantial evidence. Your credited artist is a commercial artist and a look through his portfolio shows a lot of pieces that similarly look like they are either AI-generated, or tracings of AI-generated images.
I’m also observing that the cover art is extremely large and detailed, though inconsistently so; one of the background scenes looks like a rendered, AI-generated image while others are more sketched out. But in the foreground we have 10 fully-rendered portraits of different people. Now, I assume this is a commission that you paid for but I also assume you did not pay multiple thousands of dollars for it, which is kind of the floor of how much a piece like this would normally cost. I assume you got a suspiciously good deal on a large and highly-detailed brief that most commercial artists would take multiple weeks to complete. If you did pay multiple thousands of dollars for it… I’m sorry to say but I still think this is traced from AI images, and you got ripped off.
I’d suggest asking your artist for the original process files for the image, including any reference photos as well as any sketches showing how the figures were originally drawn. I strongly suspect the artist won’t be able to produce them.