You could also have a bent float arm on the fuel sender. Pull it out of the tank and ground it with a jumper wire, then run it though its range of motion and see if it tracks with the gauge. I ran out of gas once, so I bent the arm to show empty when I had a 3 gallons still in the tank as a back up. I don't mind that it reads full a lot longer then it should, it's way better then walking!
