Use a DC voltmeter to see if it gets 12 Volts or maybe even as low as 9 to the big post on the starter motor. If it doesn't, use the voltmeter to see if it is getting 12 V to the starter solenoid.

If you're getting voltage to the solenoid, but not to the starter, the problem is the solenoid. If you're getting voltage to the starter and it won't turn, it's the starter.

If you're not getting votage to the starter, use the voltmeter ACROSS each connection to see if there is a voltage drop greater than a few tenths of a volt. Whichever connection is dropping several volts, including the solenoid contacts, is your culprit.

I did this kind of troubleshooting as a kid in my Dad's shop before I even ever heard of Ohm's Law. It's a very simple process.