If you can get to the plug, unplug it. The gauge should read full. That means that when the tank is full it should have high resistance. It should get lower as the tank gets emptier. Not sure what the resistance should be exactly though.
The only other thing you might try is to pull the cluster and wiggle the connectors a bit. You'll see the circuit board on the back is copper foil in between 2 pieces of contact paper. The tach on my '83 didn't work when I got it and after pulling the cluster for another issue the tach worked from then on.
Check for any cut wires running along the frame on the passenger side. My fuel gage went out because the wiring was nicked when my rock sliders were welded on my 85 pickup. These wires actully enter through the floor board under the passenger seat. I pulled the seat, disconnected the wires and was able to trace the wires back towards the gas tank.