It's extremely difficult for a human to discern the difference let alone computers which for all the ML are still profoundly stupid. To quote Justice Stewart i

It's extremely difficult for a human to discern the difference let alone computers which for all the ML are still profoundly stupid. To quote Justice Stewart in Jacobellis vs. Ohio: I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description ["hard-core pornography"], and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that.