Not sure about a voltage meter. You could try running into a tv audio input and turning up the volume just enough to see if you hear anything.
Does the "RF LOCK" indicator ever light up? Does moving the switch from "opt" to "coax" do anything?
Unusual that the unit does not have an optical output, only coax. The ones I've seen usually have both.
Plug a DVD or CD player into the optical or coax inputs and see if it passes the signal to a receiver.