Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the rectification lead from X1 on the PCB
Disconnect the rectification lead from the first section from the flame sense electrode

Measurement:

Measure the resistance of the lead.
Also check the general condition of the lead and its connector.
Expect zero resistance or continuity.