The information is out there, from his history in neighbourhood watch to his treatment of co-workers, etc. You also have to consider that most people don't spend their free time reporting black suspects to the police. Do I have proof that he's a racist? No, just like you don't have proof that he isn't. But sometimes you've just got to call a spade a spade.
Most people don't spend their free time... Doing what?
Do you not understand what a neighborhood watch is? On top of that, it was the dispatcher that was interested in Martin's color, not Zimmerman.