Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the orange lead from the flame sense electrode and unplug the large (20 way) connector for the right hand side of the PCB

Measurement:

Measure across the end disconnected from the electrode to the orange lead at terminal 9 of the disconnected connector at the PCB end.
Check the general lead condition.
Reconnect lead after test.
Note:
Colour of wires may vary but the terminal position will remain the same.