All I do is set the motor up on the bench or clamp "lightly" in a vise. Setup a 12 volt source. I often use my small battery charger. Ground the frame of motor with negative lead and touch one of the two power terminals on the motor with the positive.
You'll need to determine what direction its going so at first just touch the lead momentarily. If the motor groans but does not move, it could be at the end of its travel in that direction so touch positive to the other terminal to reverse the direction. Again use good discretion to not over stroke the motor too long.
This test will at a minimum determine the motor runs in both directions with no load. In the case of a window or headlight motor, the outcome could change after put under a load when the motor is actually doing work.
