There is plenty of threads on how to check the glow plug relay, check WIKI above, also there should be a thread to check glow plugs while still in truck by just pulling the plug, no dis-assembly required of the engine. If it is your GP relay I suggest one from Mouser's, the STANCOR DC relay click
HERE to go to the page, I got one and it is SUPER no more worries from the crappy NAPA, ADVANCE AUTO or such, and it is cheaper and more durable than the OEM or any other aftermarket one. Ask anyone here who has one, they should NEVER have to replace the relay again unlike the ones from stealership or auto parts store which only last about one season. Good luck.