Set Appliance State:

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

Measurement:

Test 1:
Measure the resistance between pin 1 and 3 on the gas valve.
Expect around 6.5kOhms resistance.
Test 2:
Measure the resistance between pin 1 and 4 on the gas valve
Expect around 7.4kOhms resistance
Test 2:
Measure the resistance between pin 1 and a suitable earth point.
Resistance should be at least 1M Ohm.