Successfully popping one thing and merely drawing blood from another is distinguishing between them as well. That's not distinguishing targets, though, that's distinguishing outcomes.
There's a druid wildshaped into a dog. He's a perfect dog... but his type is Humanoid, not Animal. When I cast Hold Animal, the spell has no effect, because he is an invalid target. The spell doesn't attempt to determine if he's an animal or not... it's just that if he isn't, the spell has no effect.
There's a statue painted to look exactly like an orc, next to an orc who is paralyzed. I can't tell the difference between them from where I am. I cast Magic Missile at one, and it works; I cast Magic Missile at the other, and it doesn't. The spell doesn't attempt to determine which is which; rather, if it is cast at an invalid target, the spell simply has no effect.
The spells don't distinguish the target; rather, the outcome of the casting is dependent on whether the target is valid. Belief is irrelevant; only what actually exists matters to that outcome.
-Hyp.
