On most American made cars you can test the sending unit by removing the wire attached to the sending unit at the tank and ground or touch to the tank itself. If the needle on the gauge goes to full, the sending unit is registering properly. If it does not move it could also indicate the wire has been severed between the tank and the guage.
You can momentarily ground the power wire to the sending unit and the gauge should go to full.
its the sending unit
Most likely the problem is the sending unit. A good way to verify it is to run an actuator test on the scan tool for the fuel gauge. If the gage goes through its full range then the sending unit is most likely the culprit.
the float that measures the fuel in the tank may be stuck. or the gauge in the dash may be messed up. pay for an hour of mechanic time and have them take out the float and test it with a volt meter.
Ground the power wire and have someone watch the gauge. If it goes to full then the gauge is working. Clean the ground connection and turn the key on and if it reads then it is okay. If not then you have a bad sending unit.
With a multimeter. The easy way is to unscrew, disconnect and remove the sending unit from the tank for testing. Once removed, connect the multimeter to the chassis of the sending unit and the isolated stud that was wired to the fuel a gauge. Depending on where the sending unit's arm is positioned different resistance values will result.
Run a fuel pressure test with a fuel pressure gauge connected to the fuel rail.
Either the sender wire is bad, or the sender inside the fuel tank is bad. Here is how to test it. Ground the wire that runs from the tank. This should pin the gauge all the way to full. If this happens, it is the sending unit. No gauge movement means bad gauge. Remember to turn on the key while testing.
With a fuel pressure gauge connected to the fuel rail.
Take the wire loose from the oil sending unit and then turn the key on and ground the wire to the engine somewhere and the gauge in the truck should go all the way past the highest point. If it don't then the gauge in the dash is bad are the wire going to it ( The 1 you grounded ) is broken. If the gauge is reading low oil pressure, Then unscreew the oil sending unit on the engine just behind the distributor, were you took the wire loose from. Then install a OIL PRESSURE GAUGE into it's place. Then start the engine and read the pressure, Now compare the reading on the dash gauge to what you are reading on the oil gauge.
Remove oil sending unit and replace with a mechanical gauge
You will need a fuel pressure gauge. Connect the gauge to the fuel rail and start the engine. The gauge will tell you how much pressure they pump is putting out.