I check mine cold, should be close to the same as putting the leads (+,-) leads together on the digital volt/ohm meter.
I have been using Autolite glow plugs, they are cheaper and come with the nut and washer on them. I seem to loose those little things, and go to la la land, darned if I can find them.
They last about 2yrs., I just change them out 1 at a time when one goes bad.
If bosch had the nuts & washers I'd use them.