Yes, you will need a voltmeter to check it.
1. Check voltage at the battery terminal on the solenoid - it should have 12 volts.
2. Check voltage at the small wire coming from the key switch. You will need someone sitting in the seat, clutch in and key in the start position. You should have 12 volts
3. Check voltage on the Starter side of the Solenoid. Same start procedure as #2 You should get 12 volts.
If checks one and two pass and #3 fails then you have a bad solenoid.
If check #2 fails then you have an issue between the solenoid and key switch.
If all three pass then you have an issue with either the wiring to the starter or the starter itself
I am with you on your diagnosis and believe that you have a bad solenoid