Shutterstock just banned AI generated images from others…
(the information about banning was sent to content creators by e-mails btw. just search reddit for some people showing those e-mails)
That’s the root of the problem IMO. You can potentially recognize a lot of Midjourney images due to their style… unless you use --testp… but outside of some obvious inaccuracies (like number of fingers, or eyes) you cannot really say with 100% certainty that image was generated or not. And the systems will be improved to remove those inaccuracies.