Correct, ground the wire at the tank (a, alligator clip lead works great). Turn ignition on, gauge should read "Full". Actually, it should go to top of scale past full. This means the gauge is OK, and the sender is faulty or the connections are bad. Most times bad gas gauges are caused by dirty connections back at the tank. No big surprise as they are exposed under the car.
If you ground the wire and the gauge does not move, check for voltage at the wire. Should be about 5v. The sending unit in the tank should be about 10 ohms full, 73 ohms empty. If the resistance values are way out of this range, replace the sending unit/pickup assembly.
If you ground the wire and the gauge reads less than full you instrument cluster voltage regulator is faulty. Also a common issue. This means your temp gauge is reading low too.