Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
CH Demand
Gain access to the X20 connector on the right hand side of the PCB
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

When the fan is running measure the voltage between the black wire 3rd down from the top on the RHS of the connector and a suitable earth.
Note:
You may have a black or white wire depending on whether you have an older or newer harness, both are the same and interchangeable.