You can start with shorting out the gauge only for a moment to see if it moves. Start at the gauge by shorting between the terminal that goes to the sender and a good ground with the ignition switched on. The gauge should move quickly to full. If it does you know the gauge is good. If not, the gauge is bad and needs replaced. If the gauge is good then you check the wiring to the sender by having someone watch the gauge while you ground the wire that attaches to the sender. That should, again, send the gauge quickly to full. If it does, the wiring is good and something is wrong with the sender and it probably needs replaced. Be sure the sender is well grounded. A ground wire directly from the sender to a good ground is best rather than relying on the sender grounding though its attachment to the gas tank.