To check the fuel gauge, earth the sender wire. If the gauge reads full, its the fuel tank sender unit [inside tank]. If it does not, its the rheostat on the gauge and the gauge needs changing.
Rheostat is on the back of the gauge. You could have a break in the wiring. Lift the sender unit out of the tank and move the float with ignition on and wires connected. Does it move? If not, check voltage to tank and then suspect gauge.