Generators convert rotating mechanical energy into electrical energy. They were fitted to cars until the late sixties, when alternators began replacing them.
Generators are powered by external forces, such as a car’s engine, which uses a generator belt connected around the crankshaft and the generator pulley to rotate the generator’s armature at speed. The rotational speed creates electricity that flows down the cables to your battery and other electrical devices.
To test if your 6-volt generator is producing the correct voltage, the rotor inside the generator needs to be turning, and then you simply use a multimeter to measure the output.