You can check continuity across the motor leads, any probs with windings would show up there. If it won't run when iots hooked up, check batt voltage and then check voltage at esc motor leads - they should be real close. If the voltage is diff, may be teh esc, if voltage is almost the same, its prob motor.
If its a prob with the rotor, short of one coming apart and not wanting to turn, don't know any way to check that. If motor has seen enough high temps, it can demagnetize and just stops working.