some how im sure you can see your brakes through the wheel, (if not, take the back wheels off to inspect.) so look to see if the rotor is smooth or torn up with deep grooves. you want to look at both sides of the rotor as well, because one pad can wear down faster then the other sometimes, or it can also fall apart if low enough on brake pad material.

you should also be able to see the pads. not a whole lot of them, but where they meet the rotor on the out side facing part.