We have put together some images of rotors than are either warn out or warped. You can usually visually inspect your rotors by looking through the rims or behind the tire to confirm if its time to replace them.
This rotor is warn from not replacing the brake pads in time. This brake job went from cheap to expensive real fast, replace your brake pads!!
This rotor is warped, its hard to tell except for the wavy brake pad deposits from the brake pad only touching certain parts of the rotor. If you see a wavy pattern it is a good indication of warped rotors.