If you don't have a variable rheostat then have someone reliable watch the guage as a second person removes wire at tank sender and MOMENTARILY shorts sender wire to earth. If needle flicks up to full then back to empty, chances are guage is ok. If it starts to move slowly upward toward full, and then slowly back, it indicates resistor in Guage , and you can re-short sender wire till helper says needle is rising and has almost reached full, at which point the shorted wire should be removed to open circuit to avoid damage to guage.
This could now be a great opportunity to "Flush" out an otherwise dirty fuel tank, as you GENTLY remove sender unit for testing its integrity and variable meter readings, with a test-tool like a "Fluke" or other quality Meter.