Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove flame detection electrode from its housing
Remove flame detection electrode from its housing
Measurement:
Thoroughly check the electrode lead and earth lead for signs of damage, cracked ceramic, damaged insulation and cleanliness.
Measure the resistance of the electrode lead and earth lead.
Expect continuity / zero resistance.
Measure the resistance of the electrode lead and earth lead.
Expect continuity / zero resistance.
Isolate as per TOPs P4
and refit as necessary