If people want my opinion, but they probably don't, there is no real way to tell the difference between a pureblood and a synthetic heartless. But... pureblood heartless are made when a human or animal's heart is corrupted by the darkness. The synthetic heartless are made by a machine. And, though I'm not sure if people care, more powerful heartless are made when more darkness is in a heart that's been corrupted by darkness. So, since Sora had little darkness in his heart, he only became a shadow instead of like... an Invisible or a Darkball.
Not that anyone will care about my opinion, but the weirdest movie I ever saw was Idiocracy. It was funny, don't get me wrong. But it was just... strangely disturbing.
