It is extremely simple to test a pressure, temperature or fuel gauge. They all work the same way. With the ignition on, just ground the wire that normally goes to the sensor. The gauge should read fullscale. If it does not, make sure the wire is not broken. Also check that the engine-to-chassis ground is in good condition. I very seldom find a bad gauge.
Fuel tank sensors will often read low because of a poor ground between the gas tank and the sensor. I always put a ground strap directly from the chassis to the gas tank, otherwise you are relying on the screws that are used to secure the gas tank to provide the ground. These screws rust and then the ground goes away.
If the gauge works then the problem is with the sensor. SIMPLE!