How is the easiest way to test a starter for a 1979 chevy?

Take it out and have it tested on a test stand. Barring that using a battery of known quality.

Old mechanics will tell you they used to take a big hammer out to the car and a helper to turn the key.
Or a gentler way is a test light or a volt meter on the solenoid wire at the starter with key in crank mode. Should be battery voltage down there.
Engine grounds are just as important. What's it doing?

Take the starter motor off and take it to a auto parts store. Most (Autozone, NAPA, O'reilly, etc) have testers and will test it for free. If it tests bad then you can replace it right then (there is a core charge and you already have it out). If it tests good, then you know to start looking into other areas of the starting system.