Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove flame sense electrode
Remove flame sense electrode
Measurement:
Measure the resistance across the electrode.
Expect continuity / zero resistance.
Also visually inspect the electrode for cleanliness and good general condition i.e. no cracks in the ceramic, distortion or damaged insulation.
Is the resistance measured as expected and the electrode and lead in good condition?
Note:
Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary.
Expect continuity / zero resistance.
Also visually inspect the electrode for cleanliness and good general condition i.e. no cracks in the ceramic, distortion or damaged insulation.
Is the resistance measured as expected and the electrode and lead in good condition?
Note:
Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary.
Isolate as per TOPs P4
and refit as necessary