Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
NO demands
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the DC voltage between the Red at the DC fan connector (4 wire) and a suitable earth.
Expect approximately 22 VDC.