The output frequency of a generator is measured in Hertz, which refers to the number of alternating current (AC) cycles per second. The standard frequency for electrical service in the United States is 60 Hertz. The RPM of the engine that powers the generator is the factor that determines the frequency, and the generator owner's manual contains instructions on how to adjust the engine speed. Before you can adjust the frequency, you need a way to test it. High-end multimeters include this function, but a cheaper and easier way to measure frequency is to use a Kill-A-Watt-type household load meter.
Plug the Kill-A-Watt meter into a standard 110 volt outlet either directly on the generator, or connected to it.
Start the generator and allow it to reach full operating temperature. Plug a small heater or an incandescent lamp into the generator to provide a load.
Press the "Hz" button on the Kill-A-Watt meter. Read the frequency directly on the LCD screen.