Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across the white and black wires on the white 3 pin connector to the DHW flow switch.
Expect approx. 5V DC.
Note:
Plug shown disconnected for clarity but should remain connected to the flow switch.