Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect flame sense lead
Remove flame sense electrode

Measurement:

Check the flame sense electrode is not warped or damaged and that there are no cracks to the ceramic or electrode.
Carry out a resistance check between the ends of the electrode.
Resistance should be short circuit (zero resistance).