Are you talking about the plugs themselves?
If you put one lead on the terminal and the other on the body (the part with the threads), there should be 2 ohms or less. If it's wide open, the plug's bad.
Hope that helps some...good luck! BTW, one morning in January, my van wouldn't start...turned out that 7 of the 8 glow plugs were bad. I put new Beru plug sin and it started without even thinking about it, even from being dead-cold...I was impressed
