It's not clear to me from your description that you are determining ANYTHING about the function of the plugs but may be monitoring mostly the effect of the couple of resistance wires (the serpentine shaped wires that connect between a couple iof the plugs) which generate and dissipates into the engine compartment more than half of the heat the system produces.

When the system is working properly and energized, there is a little more than a one Volt voltage drop across each glow plug (from the top to the bottom connection onn the plug) when measure with a DC meter. The voltage drop across each plug will be about the same. If one is much higher than the others, that will be the bad plug.

The actual voltage measured across each plug will drop some with time as the plugs heat and current drops. You may have to activate the system several times to measure all the plugs as it will cycle off after 30-60 seconds.

Marshall