Bench testing.
Connect a set of jumper cables from a battery to the main cable terminal on the starter and the ground of the jumpers to the starter nose at one of the bolt holes. With the front of the starter facing you use a screw driver to jump from the large solenoid terminal to the small one to the right of it. This will energise the solenoid and spin the starter. It should jump right back to the end of the shaft and stay there until you remove the screwdriver. All it really tells you is the drive is fully engaging and the motor is spinning. It won't duplicate the starter's performance when cranking the engine but if the drive is really shot you should notice a problem.
Oh yeah, hold onto the starter when you engage it.
