You will need a multi-meter that can measure Hertz or frequency. Attach the leads to the output voltage connections and you should read about 61.5 Hz no-load. At load, you should measure about 58.5 Hz.
To change the speed on a BG or NH series unit, you must adjust the speed spring that is under the black cover on the top of the engine. Early units had a square hole in the cover (two wrenches) and later models had an extended shaft (Phillips screwdriver), like a speedometer cable, that terminated in the vicinity of the carbruetor.
If the genset is hunting GET A "SERVICE MANUAL" for your unit and follow the directions explicitly or you will have trouble.