conversion math

Precisely.
The math of electricity deals with absolutes.
The more precise one is with the input data the more precise the answer will be.
Advertising, on the other hand, has a tendency to exaggerate. But you know this.
Key words here are - rated output.
 If the manufacturer of this product was able to achieve this "rated output" then there would have to be a test condition. Was the test condition a standard among generator manufacturers? What are the conditions?
 
A generators output is directly related to its input speed and torque values as well as any load that would be placed on it which could have an effect on internal temperatures and so on. All of this would appear to be picking nits but, you wanted to know, and the best way I know how to know an unknown is to know what the unknown is and then form an hypothesis. Then experiment to prove or disprove that hypothesis. After all Micheal Faraday didn't invent the very first electric motor sitting on his keister you know.
http://www.rigb.org/heritage/faradaypage.jsp
Not that I think you are prone to sitting on your keister or anything, its just a figure of speech.
Anyway, you will need; an amp meter, a volt meter, an rpm gauge, a power source (to spin the generator,) and a torque value gauge. You know what, on second thought, it would easier to preform another well known mathematical step that electricians use known as de-rating, which is to assume that it will not put out its advertised rating.