Set Appliance State:

Mode selector switch to Off/Reset position
Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance
as per TOP's P4

Disconnect one of the purple wires from the DHW flow switch (actuator)

Measurement:

Measure the resistance across the terminals of the DHW flow switch. Resistance should be open circuit.
Turn hot water tap On.
Measure the resistance across the terminals of the DHW flow switch. Resistance should now be closed circuit.
Reconnect wire after test.