Set Appliance State:

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

Observation:

Measure the resistance between the disconnected spade connector
and pin 4 (white wire) on CN1 on the ignition PCB.
Also visually inspect the lead for good overall condition.
Expect continuity / zero resistance.