One of the wires on your generator set switch is the "B+" wire. It provides 12 VDC power from the genset whenever the unit is running. This normally lights the LED on the rocker switch but it also is tapped into to run the hourmeter. If your switch or hourmeter are not located on the dash, you should be able to drop down to the genset itself or the switch and run a 12 volt wire from it to the dash, where you can mount a 12 volt LED or pilot lamp.
The other option is just to take the muffler off. Then you'll always know when it's running.