Best way would be to do voltage drop test across the solenoid. Connect a voltmeter between the two large terminals of the starter and have someone hit the key. Anything more than 1/2 a volt with the starter turning the engine and the contacts are bad. This is a good test you can use to find bad grounds etc because it checks how good the contacts are under a load. It works well for things like battery cables etc. You can measure voltage between the negative battery terminal and the engine block during cranking or between the positive terminal of the battery and the post on the starter to check the integrity of the positive cable.
