Gas gauge / sending unit testing

Help if you can. I cannot get my 62 Valiant gas gauge to work. It is a new Bosch. I replaced the original gauge (which didn't work either...) The shop that redid my fuel tank says the sending unit is new...is there more to testing it then looking for voltage on the blue wire that runs directly from the sending unit to the gage and a ground? I REALLY don' want to drop the tank...thanks.