Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the lead from the gas valve

Measurement:

Measure the resistance between the pins of the gas valve.
The resistance should conform approximately to the table below:
PinsResistance
1 and 36.6k Ohms
3 and 4 900 Ohms
1 and Earth >1M Ohm

Note:
Pins are numbered on the gas valve.