Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove flame detection electrode from its housing

Measurement:

Thoroughly check the electrode for signs of damage, ensure it is clean, not warped, ceramic insulation is not cracked and continuity is satisfactory.
Note:
Before refitting the electrode inspect the sealing gasket and replace if damaged.