download aqua's keytest app and type all the keys. If they all registered by the key press, then you are good to go.
EDIT: did i just repeat what rip said?... oops. Oh also, listen for the "ping" noise coming from the keyboard when typing. If it pings, its defective.
EDIT2: OK I take that back, don't listen for the "ping" sound. It was meant to be a mean prank. Just make sure it performs in KeyTest app.