ORIGINAL: RCKen
Yes, they can go bad and still glow. The coils have a plating of platinum on them. If this coating is gone the coil can still glow, but the engine will be difficult to run, or not run at all. If you change out the plug and it runs good with a new plug that's probably your best clue that they plug is bad. Even though it still glows toss the old one, it's no good.
Ken
i dont toss mine, i put them in my r/c boat which runs at WOT all the time, the glow plug will last a little longer. my problem w/ the plugs is i can never get a reliable idle. but in a boat this isnt a problem (very important on a plane if you overshoot the runway). also if the boat dies in the lake its not a big deal. if a airplane dies right after takeoff you can get into big trouble. so if your in r/c cars/boats/maybe spad airplanes. keep the plug, if not trash it