Well, glowing plugs is only half the matter. There is a coating (a plating) of a catylist, Platinum, on each of the coils. This is an absolute must for glow plugs to operate. They may glow, but if that coating is not there, well, they just won't work.

The only real way I know of checking them to see if they are any good is to fly with them. Not just start up the engine, but to fly with them. If you can fly, usually, the plating is ok.

CGr.