Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the electrode assembly from the boiler

Measurement:

Inspect the spark and detection electrodes, they should be clean and the ceramics should be free of any damage or cracks.
Make sure the gap between the spark electrodes is approximately 4mm.
Check for continuity (zero resistance) between the ends of each of the electrodes.