Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the Coding Resistor from the PCB

Measurement:

Measure the resistance across the coding resistor terminals.
The resistance is different depending on the type of gas the appliance is set up for.
Expect approximately:
Natural Gas: 1.1 KOhms
LPG: 750 Ohms
Note:
Ensure the coding resistor is refitted correctly after the test.