Set Appliance State:

Mains Power Off
Isolate Mains Power to appliance
as per TOP's P4
Set Multimeter to Continuity Set meter to measure Continuity
Disconnect flame sensor lead from electrode and PCB.

Measurement:

Inspect electrode lead for good general condition, signs of shorting and carry a continuity check on the lead.
Inspect flame sensor electrode for good condition and no signs of damage.