Another less accurate way is to nail 3 pieces of wood together in a foming a U shape with a v notch cut in the top of each upright. Drill a hole in each upright slightly larger than a nail, set the axle in the V notches and slide a nail through the upright so it touches the rotor, slowly turn the wheel and if the nail moves or you see daylight between the nail tip and the rotor it's warped.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!