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.
Resistance should be approximately 6.5kOhms.
Test 2:
Measure the resistance between pin 1 and 4 on the gas valve
Resistance should be approximately 7.4kOhms.