No starter

Truck not starting. Had battery checked, it's good. Previous owner had dickered with the ignition switch in the steering column, and the ignition solenoid on the frame. Truck previously would start, but only when in neutral or (get this) reverse. Sometimes I hear the starter solenoid click, sometimes not. I know the leads from the battery and to the starter (the heavy leads) are correct on the ignition solenoid, but not sure about the the two small gauge ones to the starter and ignition. How can you check those without tearing everything apart? And any other suggestions to check?