How can I tell if the starter solenoid is bad?

Check the small wire on the starter solenoid, which is the actuating wire from the ignition switch, by having a helper hold the ignition key in the "Start" position while you probe the small wire for power. There should be 12 volts every time the ignition is in the "Start" position. If it is hard to probe the wire on the solenoid, the wire can be removed for testing. If there is power to the small wire and power to the large wire from the battery, the starter is bad.