Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Gain access to inner combustion chamber
Selector Switch to Standby
No Demand
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Measure voltage between the flame sense probe and the earth terminal on the fan.
Expected voltage approximately 68 VAC.