It is difficult to check the heater, as the plastic Connection is both the water in fuel sensor, and a Thermostat, that turns heater on and off.
You can check to see that voltage is getting to the connector, but you have no idea if the sensor is passing the voltage inside the bowl, or if the heater itself is bad.
I have owned a few PSD, and when I bought the last one, I noticed that when cold, it didn't run as good as it should when it was cold, and was effected by #2 fuel far more than it should have been when the engine was warm, but the temps outside COLD.
I pulled my entire bowl, and tested the sensor by ohming it, then putting it in the freezer and ohming it, to make certain it was making the connection when it was cold....it did....I ohmed the heater, and it was open. Tested further, and it was the diode on the heater that was bad.....Not sure why it needs a diode, but I replaced the heater element with a new one....could have just bypassed the diode...
Most people will NEVER know if the heater is not working.....But being I am in tune with my PSD, and have owned a few, and am a mechanic, I recognized the symptoms.
If it weren't for me liking to run straight #2 in the middle of winter, I doubt I would ever have noticed.