Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the leads from the spark electrode
Carefully remove the electrode from the combustion box door

Measurement:

Measure the resistance between the tip of the spark electrode and the lead connection point.
Expect continuity/ zero resistance.
Note:
Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary.