If you have a spare sender find out what Resistance is required to make your gage sweep full scale then check the sender against that. Wally
You can also do this with a $2 ~500 ohm potentiometer (aka "pot" aka variable resistor) such as this:
T7YB471MB40 Vishay/Sfernice | Mouser
Turn the car on with the potentiometer temporarily wired in place of the sender. Twist the pot around until the gauge reads empty, disconnect the potentionmeter, and measure the resistance between the same two terminals on the pot. Then do the same after getting the gauge to read full. Those two resistance figures are the key numbers for the gauge. If you're planning on installing a Centroid or similar sender and want it really accurate, take measurements at 1/4, 1/2 and 3/4. and gives to them when ordering.
Typical figures are (courtesy of Summit Racing)
0-90 Ohms - Works on most GM cars, 1965 to present
73-10 Ohms - Works on Fords prior to 1989 & most Chryslers
240-33.5 - Industry standard, works on many popular cars
0-30 Ohms - Works on most GM cars prior to 1965
16-158 Ohms - Works on most Fords, 1989 & newer
I dont think you'll find that range pot at Radio Shack, I'm sad to say. I think the closest they have is 5,000 ohm, which probably does not have the resolution you need to do this properly.