Since you do not specify which starter motor model I give you an overview: coals, bushing wear, mass testing of the rotor winding and the field winding if it carries it because there are starter motors that the inductor is magnets permanent, short-circuit winding test, check that the drive pinion locks or locks to one side and releases to the other. Check continuity in solenoid windings or automatic start-up, check the good condition of the drive pinion fork, check the good condition of the insulators.