This isn't really a good way to spot it. Theres a lot of tools that artists use that can end up with digital paintings doing weird artifacting like this, one I can think of are custom brushes or people that collage to make their art. Also, theres a whole new brand of artist that uses AI in a workflow to make their images faster. They might sketch something out and then have ai fill in the most time consuming parts (such as a head of hair) and it will all look very believably not AI. It basically is just collaging but way faster.