Poor man method - run the motor in the car with the wheels off the ground at 1/4 throttle for a minute or so. Watch the brushes - if they spark a lot at first, keep going until the sparks settle down. That's a sign that the brushes are seated.
BTW, P2K2 motors need to be geared correctly, if you overgear they get hot quickly. I've had and seen several that the magnets came unglued and fell out of the can onto the armature after getting too hot.