Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
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.
Expect a resistance of 1 MOhm or greater.