Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the spark electrode from the ignition lead assembly connector

Measurement:

Carry out a resistance check on the electrode to a suitable earth point.
Expected resistance above 1MOhm (1000kOhms).
Also inspect the electrode for good general condition and signs of shorting.