Install the spark plug of the generator - start the generator - look to the fuel-level-manometer "for example: max level" - after some times the fuel-level fall down to "for example: half level" - stop the generator - remove the spark plug - the fuel-level will switch now from "half-fuel-level" to "max-fuel-level" - install the spark plug and start the generator again - the fuel-level will fallen down to the real fuel-level (half-fuel-level).