You may need a volt meter. Check each one in turn.
'Check to see what the resistance is of the glow plug. If you do not have an auto-ranging meter, try other resistance setting to verify you measurement. If the meter reads an open or infinity measurement (on Fluke meters, it will read OL), then the plug is defective. If the glow plug shows resistance that is approximately less than 5 Ohms, then the plug is not defective'.
'The removal procedure and the resistance specs for glow plugs varies depending on the vehicle and glow plug used; for specific information, consult your particular vehicle's manual.'