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 Blue wire at the fan DC connector (4 wire) and a suitable earth.
Expect approximately 2 - 4 VDC when the fan is running.