Test for voltage as follows:
1) Battery voltage (measure across the battery posts (not the terminals on the cables). 12.6 volts = good
2) Measure voltage between engine block and the point where the positive battery cable connects on the engine. 12.6 volts = good
3) Place positive test lead on the small solenoid terminal fed by the ignition switch. Turn key to START. 12.6 volts = good. If no voltage or very low voltage, bad ignition switch or wiring. Verify power at "B" terminal on ignition switch.
4) Place positive test lead on the large solenoid terminal feeding the starter motor. Turn key to start. 12.6 volts is good and at that point starter would be suspect.
If 12 volts not present at starter side of solenoid, THEN replace solenoid.
The trick in troubleshooting electrical is follow the juice. When you lose it you found the problem. The path is positive terminal on battery, to solenoid, out the solenoid to the starter. The solenoid is energized by the ignition switch which gets power from the battery, sends it to the "S" terminal from the "B" terminal on the switch.