How do you measure watts output of a generator?

To do it yourself you will have to be familiar with an amp meter, and ohms law, and be able to trace to main wire leads from the stator windings. I'm sure that an electric motor repair shop would do it for you cheaply. Have you closely examined all tags and labels on the unit? Take the cover off and look real good. UL requires that all manufacturers properly tag all equipment with capacities.

A quick rule of thumb, take the amperage of the breaker that protects the plug on the generator. Multiply this amperage by the voltage that comes from the plug. This will give you an indication of the power in watts that this generator should be able to produce. Watts = Amps x Volts.