Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Check condition of flame sense electrode and lead assembly.
Disconnect flame sense lead at inline spade connector and check resistance between electrode tip and disconnected end of lead.
Disconnect flame sense lead at inline spade connector and check resistance between electrode tip and disconnected end of lead.
Isolate as per TOPs P4
and refit as necessary