Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Disconnect flame sense electrode lead from the flame sense electrode and the PCB (connector J12)
Disconnect flame sense electrode lead from the flame sense electrode and the PCB (connector J12)
Measurement:
Measure the resistance between the ends of the flame sense electrode lead.
Resistance should be short circuit (zero resistance).
Inspect lead for good overall condition.
Resistance should be short circuit (zero resistance).
Inspect lead for good overall condition.
Isolate as per TOPs P4
and refit as necessary