Edited with BC's assistance:
1. Plug in for 3-5 hours...Is the Valve cover Toasty Warm...It works just Fine.

2.More difficult...With DMM set to "Continuity Test" apply leads to poles of engine block heater plug.[Then you can do #1 to double check]

3. If you stand Very Quietly and lean over the engine,You'll hear the Coolant
kinda gurgling.

thanks BC

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'84 300SD sold
124.128