How to tell if the fuel gauge works

Professional scan test.

To supplement Pushrod's response -- a good diagnostic tool like Ford's IDS testing system can "active command" a gauge to make sure the gauge moves when commanded by a simulation of the fuel sending unit's information. If the gauge responds with the IDS, then you know that it is not the problem. The fuel sending unit in the tank as part of the fuel pump or the wiring could be suspect.