Fraid this one's different. Is it hard to check the heater plugs? I'm told they can be troublesome to extract.
Some come out easy if you have the right size deep socket, but some seize in the cylinder head, so if they don't come out easy do not force as can snap costing loads to fix
Depending how mechanically minded you are and what testers you have .
If heater plugs are still in the engine , pull the plug terminal off and using a Mulimeter set to Ohms one wire of tester to earth on engine the other to the heater plug terminal , you should get a continuity reading, if not reading heater plug is probably duff.
Test the supply between plug and engine using a voltmeter setting on DC volts on your multimeter you should get a reading up to around 12 volts depending on the type of heater plug recommended for your vehicle (some are rated at 11volts , some at 4.4volts etc) it may only show this reading when engine cold and for a few seconds when key first turned on, not to start position, just ignition lights.
If heater plugs come out easy and on inspection have 11volts written on them , if careful you can test using a set of jump leads connected to a car battery.
Note only lightly touch to terminal and only for a few seconds, it should glow bright red quickly, then remove connection to battery.
As I have a Professional Amps and Volts Wilkson Test Set from years ago I can test the for amps as well as that meter reads up to 30 Amps , normally a heater plug will draw 8-10 amps, which will probably destroy a cheap multimeter