Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Electrically disconnect flow switch at the inline connector

Measurement:

Measure the resistance across the DHW flow microswitch switch white and black wires (at the inline connector end that goes to microswitch).
Resistance should be open circuit.
Turn hot tap ON.
Measure the resistance across the DHW flow microswitch white and black wires (at the inline connector end that goes to microswitch).
Resistance should be closed circuit.
Reconnect inline connector after test.