[cheshired5]

Multimeter set to DC Volts. 20V if it's a manual ranging meter.
Red multimeter lead into the glow plug connector, black multimeter lead on the bare metal of the engine then cycle the ignition but bear in mind the coolant temperature as mentioned previously.

It's worth saying that Dice and Vida makes this job very simple as it can activate the glow plug relay at the touch of a button and of course offer detailed diagnostic information.