your service manual has all the information you need.
basically you need to know how the system works. Once you know that, you can figure out which components to test, and from there you'll eliminate the known good ones leading you toward the bad one(s).
"Normally" the controller is bad but I've put a bunch of relays on too, and a few temp sensors. And plenty of wiring repairs.
So it "could" be any component that has to do with the glow plug system. That's why you need a work shop manual, a volt meter, and some basic mechanical and electrical knowledge. But fear not, you CAN do it.