Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove both ends of the flame sense harness, from the electrode and from X501 on the PCB

Measurement:

Measure the resistance of the flame sense harness lead.
Expect continuity / zero resistance.
Also visually inspect the condition of the harness.
Is the resistance measured as expected and is the harness in good condition ?