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 Black at the fan DC connector (4 Wire) and a suitable earth.
Expect approximately 16 VDC.