Justice is right.

Diagnose a starter on the ground with a pair of jumper cables. Black to motor frame. First, red to upper solenoid terminal, then jumper wire to small contact. gear should jump out, then motor runs. If one and not two, solenoid is bad in motor contacts (most common starter failure). If no gear out, solenoid is bad in windings (rare). Test for a good motor by going straight to lower solenoid terminal which is connected to motor windings and motor should spin, but gear will not extend.

Most common fault is arc corrosion on copper disc in front end of solenoid, caused by many contacts with hot battery current. This is why whacking a reluctant starter works, as it gets the tiny fragment of corrosion out of the way and lets motor current flow. The solenoid clicks, but the motor doesn't turn. Almost all starter failures can be fixed with a new solenoid, and cheaply...