Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Remove outer combustion chamber
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the fan run continually?
Note:
It may be possible to determine whether the fan is running without removing the combustion cover, use engineer judgement.