I forgot to add, to compare glow plugs, you have to look at it when it is connected to the glow ignitor.
Pull the plug from the motor, put it into the ignitor and look at the coil...Is it glowing red evenly? Are all the coils glowing? if the first coil (closest to the outside) is not glowing it is probably bad.

Look at the pic below to get some good info on plugs.