Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
CH temperature control to max
Ensure flow and return isolation valves are open
Set selector switch to ch/dhw
Attach 1x temperature clamp to the flow pipe base of boiler
the other attach to the return pipe at the heat exchanger
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the boiler to run on ch demand for a few minutes.
Does the flow pipe reach a greater temperature than the return pipe?
Note:
Use a FGA (Flue Gas Analyser) to measure this temperature difference.