Set Appliance State:

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

Measurement:

Test 1:
Measure the resistance between pins 1 and 5 on the gas valve.
Resistance should be approximately 4.2kOhms.
Test 2:
Measure the resistance between pin 1 to a suitable earth point.
Resistance should be above 1M Ohm on each of the 4 checks.