If you are just troubleshooting to see if the problem is in the gauge or the sender...
Pull the wire off the sender and connect a 50 Ohm (or 30, or 20, or 100 Ohm, whatever is easy to find) resistor via a wire with alligator clips to a good ground in the car.
Your gauge (if working) will swing to full scale (or close to it).
Go from there. It's a pretty simple - low resistance = higher temperatures.
Good Luck -
PS> Data from SOBill, at Pantera Place.