You should check each plug while installed to first see if it's getting power with the glow light on. I get approx 11.3 at each of my glow plugs when she's glowing., but that test is to first check power to the plug only.
Next you have to get an OHM meter to check the resistance on each plug. I read that they should be between 1.7 and 3 OHMs each. Is this right anyone?
I know that if they are bad they will not show any resistance and just show infinity, as if you weren't touching them at all. Luckily, they are cheap, so I got replacements at AutoZone for 11.00 each and now she fires up at 30degrees f running 100% soy biodiesel with some DFS additive.
CYA,
Adam