the 'normal' fuel sending unit is a variable resistor driven by a float on the end of the arm. They are usually located on the top of the tank.
the quick check for the gauge is to remove the sender wire at the gauge. momentarily connect a jumper between the sender and the ground wire on the gauge...and the gauge should deflect to full scale (pegged). a better test is to use resistors to replace the sender. 33 ohms (sender to ground) should give you indicated full scale and 110 ohms should be 1/2 tank, and 240 ohms should be empty.
If the gauge works fine, you can use an ohm meter to measure the gauge - between the sender terminal and the mounting plate (which should be at ground potential) - it should be between 240 and 33 ohms, depending upon how much fuel is in the tank...if on a trailer, you can move the rig to a hill to get the float to change position.
Many times, the wiring (connections) between the gauge and the sender are in poor shape...