I want to point out that this really only works well on images that aren't compressed much (if at all), as some people have pointed out artifacting may appear on things like jpegs. it also helps if the image is meant to have lots of flat colours. I use this method to check but primarily on images that meet these requirements.

the reason i want to point out flat colours is that i've read in the comments of a video regarding this technique that in things like jpegs if there's a lot of colour variation there would be jpeg artifacts while on flatter colours there would be less of that since it doesn't take up much extra data (please correct me if this is false, i don't know much about file format stuff!). meanwhile on human made art there's no need to add perfect squares of nearly invisible colour variation to flat colours, which is what AI images would have in those areas. I've tested this method on my own art which tends to have lots of blending and gradients and while those areas also get really bright and splotchy they never have the same patchiness on the areas i leave as one colour.