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 and lead for signs of damage, cracked ceramic insulation, continuity and good general condition.
Refit electrode after test.
Note:
Any gaskets or seals disturbed during this test should be checked for integrity and replaced if necessary.