get good quality OHM meter....Lowes/Home Depot/ Any good electrical supply house - about 25 bucks.
Place on Ohms - take leads and touch each other - reading should be zero...or .001 something.
Unhook glow plug leads from glow plug......remove the "wire"
touch top end of glow plug with one lead and NEG battery with other while on OHMS.
YOU know it is good when it shows a number close to 1 ohm, 2 ,3 ohms I would still say it is good......anything that does NOT read, or a very high number....glow plug is bad.
You cannot test a glow plug if it is still hooked up with the lead on top
A less accurate way is to have a infrared thermometer......point at base where it goes into head, record reading - turn glow plugs on for about 30 seconds and read again.....you should see a rise in head temp at that specific spot.