The wire on the back of the generator with the red rubber cap is the field terminal. You should have a resistance reading from it to ground of around 4 - 10 ohms. If you get an open circuit, irritate the belt and pulley a little. The carbon brushes commonly make poor contact when the rotor isn't spinning. You can also make that measurement from the "F" terminal at the voltage regulator's plug. That will include that wire in the test.
To do the full-field test you need to apply full battery voltage to that field terminal. The easiest way to do that is to unplug the voltage regulator and jump the "I" and "F" terminals in the connector together. Those are the two outer terminals. Do that while the engine is running, but just long enough to verify it charges or it doesn't, and don't raise engine speed. All you might damage from excessive voltage is the ignition system control box and any lights that are turned on. On newer cars with all the computers, they can be damaged too from excessive voltage.