Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the connector from the gas valve

Measurement:

Test 1:
Measure the resistance from pin 1 to 4 of the gas valve solenoid.
Expect a resistance of approximately 58 ohms.
Test 2:
Measure the resistance between pin 1 and a suitable earth.
Expect a resistance of 1Mohm or greater.
Are the resistances measured as expected?