If my memory serves me right, take the pos lead of a volt meter and connect to the pos battery terminal, then take the neg lead and connect to the POS terminal of the starter solenoid. The reading should be less than 1 volt with the circuit energized. Anything more indicates resistance in the circuit.

Hope this helps.

Good suggestion, quicker than mine.

Maybe the Starter Brushes are worn out.
I think in the end the Starter will need to be pulled and if you want taken apart so that the parts can be cecked.
That is assuming the Electrical Connectoins are all OK and a good Battery.

More data:
I have a digital voltmeter in the cigarette lighter. Key on voltage is 12+ volts. When it 'clicks' and starter doesn't spin, voltage drops to 10+ volts. I think this is another indicator that starter is getting power, just not spinning.