Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect leads from Gas Valve

Measurement:

Test 1:
Measure resistance across the two outer pins on the gas valve.
Expect a resistance of approx. 4 kOhms.
Test 2:
Measure the resistance between the centre pin of the gas valve and one of the outer pins.
Resistance should be greater than 1MOhm (or O.L)