Here are signs of bad rotors:
Pulsating brake pedal
Intermittent brake noises
Grinding when hitting brakes
Screeching after brake pad installation
Out-of-round rotors
Deep grooves or score marks
Cracked rotors
Blue discoloration
Corrosion
Strong chemical fumes
Wobbling or rotor runout
Lopsided, inconsistent braking
Oversensitive/insensitive brakes
Longer stopping distances