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, corrosion, cracked ceramic insulation and also that it is clean.
Also measure the resistance across the electrode, expect continuity / zero resistance.
Refit electrode after test.
Note:
Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary.