The best way to test it is as JMOR suggested. The solenoid is an electro magnet with a plunger in the center that connects to an arm on one end to engage the bendix and the other end has a large copper washer that contacts the two lugs inside the solenoid. If when you do the test, the bendix throws out and the starter spins your problem is somewhere else. If the bendix engages but the starter doesn't spin the copper washer is not making contact with the lugs, often due to arcing. If nothing happens the electro magnet has an open circuit. Some of the early ones could be taken apart and you could clean the contacts and reassemble to get more life out of them.