Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove ignition/flame detection electrode

Measurement:

Inspect the spark electrode assembly for damage and continuity, ensuring the ceramic is in good condition.
Note.
For continuity test, remove oxide from a section of the electrode, ensure a good connection is made to eliminate a false reading.