Set Appliance State:

Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
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 8 VDC when the fan is running.