The plugs themselves are pretty simple. Basically a very low resistance (1.5 to 6 ohms depending on variables) nichrome wire heating element. In this example figure a 24 volt system and a 3 Ohm glow plug. Apply the Ohms law Voltage (V) = Current (I) * Resistance (R). and that this one plug draws 8 Amps. So, one could figure if all plugs are a working 8 Amp each times 8 plugs equals 64 Amps. (average this simple case)
Do it again with say 1.8 Ohms per plug and we get 13 Amps each plug. Add it up, now 13 Amp times 8 plugs for a whopping 104 Amps. Both these examples are real world conditions for glow plug current draw and both trucks glow plugs are converting electrons to heat. The later example is getting there a lot quicker.
Time is the next variable. Think of it like a toaster. Like yours light brown, leave it in a short time. Burnt on the edges, a bit longer.
My quick test is easy, but requires an clamp on Amp probe. I can take a reading of all of them by clamping on to the common feed out of the control box and flip to run, reading total current (like above big Amps), or individually (better check, each Amps) to each one and add up the currents for a total. Any with no current = bad plug/s. Takes more time to add it up (toes and fingers) and cycling to RUN. Total time to test all 8 less than 2 minutes. Note. control box need to be working and not timed out. That would be a bigger problem.
OR
Multimeter test just as easy. Pull one plug lead and determine if 24 Volts is present at the lead (poke positive probe up into cup contact and other to ground) Have help select to RUN while the waiting to start light is on during an attempted cold start. (quick sample 3 seconds. You have voltage or not, done) No voltage, bad box likey. Got voltage... pull all plug leads and measure each plug resistance to read in the ballpark 1.2 to 6 Ohms. Much more that 10 Ohms, replace.
The resistances of each plug won't be all the same, but should be balanced around an average.
I read here many times "truck tries to starts, smokes, ran good last week, and / or won't run now" Sure there a many like variables, but for starters (pun) knowing and understanding this basic electricity of glow plugs will eliminate the guessing / trouble shooting. Got to start some where and this is the least dirty and smelly.
Be sure hood prop is locked in place, CAMO
