Glow plugs should ohm to about 1 ohm each, and yes, you can ohm them through the VC gasket. They will be the outermost two pins on each connector and you'll just measure between them and a good engine ground. Generally if they're bad, they will ohm out open or a much higher reading, but 0.8-1.5 ohms is normal.

 

Thanks DIY. Two outermost on each end of the plug eh? Good information.