Set Appliance State:

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

Measurement:

Measure the voltage between the grey wire 4th down from the top on the RHS of the connector and a suitable earth.
Note:
You may have a grey or yellow wire depending on whether you have an older or newer harness, both are the same and interchangeable.