DISCLAIMER: I have never been in possesion of, or desired to be in possession of, nor seen, child pron. I still don't, and I will never attempt to be in possession of or see child porn.
But still, how does the consumer (a.k.a. disgusting pervert) tell the difference?
Well, that's half the point of the SCOTUS ruling: It doesn't matter. The point of child porn laws isn't to prevent sickos from getting off on certain kinds of pictures, it's to prevent innocent children from being used sexually in order to create the pictures. If the entire graphic is fake, no child is harmed, thus there's no point in it being illegal.
(This does lead to another question, of course: Would the open availability of computer-generated child porn be more likely to allow said sickos to get their kicks without resorting to making moves on actual children, or would it just make them even MORE likely to go after real children? I personally believe the former is true, for the same reason millions of men read Playboy and Penthouse without turning into rapists. But I'm not aware of any legitimate studies on the issue.)