Remove the plug, connect it to your glow driver and see if it glows. If it doesn't, something's wrong - either the plug is bad, or it's not getting current from the glow driver.

If it does glow, It could still be bad, although that is pretty rare.

To put the glow plug back (Or put in a new one), first make sure the copper washer is there and tighten it just past "Good and snug". Tightening too much can strip the threads in the head and then you have real problems.