Xemnas is easy to spot because of his big black coat. It's a trademark of the Organization. He's also got really long white hair and orange eyes.

Vanitas is a little bit trickier because he looks so much like one of the good guys, when he's not wearing his helmet. He spends a lot of time with this silver helmet on, but when you can see his face he's got black spiky hair and gold eyes. Those are the easiest ways to spot them.