You should check it with the glow plug connected. Takes a little bit of a trick with a thin strand of wire to do it, but not a biggie. You should see around 6 to 7 volts at the #2 cylinder glow plug on a cold engine initially, climbing towards 11 as they heat. As long as your battery voltage stays around 12 volts, this would indicate you have a healthy set of glow plugs.