Glow Plug Tests with Manual GP Button

Saw smoke and an orange glow near the glow plug closest to the cabin on the passenger side.
At first it was thinking a fire but I’m wondering if a glow plug burnt out. I checked the troubleshooting guide (the old looking one) but it doesn’t mention a button you have to hold to warm the glow plugs. Does that mean this manual glow plug button (circled in the photo) I have was a mod?

What should I check or test prior to putting the new AC 60g glow plugs in if it has a manual glow plug button? I don’t want to swap all 8 and just have them burn out immediately if something else is wrong.

I know there should be the resistor and it looks like it goes to the solenoid but don’t they have to be under load to drop voltage? I can run a multimeter but don’t know what to check before swapping plugs. Thanks!

Yes, this is modification.
Just remove plugs and test them separately. 12V.

Myself when checking glow plugs it's easier to leave them in. I disconnect the wire take a test light and hook the alligator clip to positive on the battery and then with a point touch the end of the tip of the go plug and the light lights up the glow plugs good.