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.