This is from my 96 service manual:
1. Remove the electrical connector at the side of the fuel heater.
2. Using a ohmmeter, check the resistance across the two terminals on the side of the heater. Resistance should be approm. 1 ohm (cold) to 1000 ohms (warm).
3. With the elect. connector still unplugged from the fuel heater, check the electrical operations of the fuel temperature sensor.
4. Using an ohmmetr, check the resistance across the two terminals in the pigtail wire harness coming from the fuel temperature sensor. The sensor circuit should be open if the fuel temp is above 80 degrees and closed if the temp is below 40 degrees.