Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect DHW flow switch at its inline connector
Note:
DHW flow switch is located behind the water to water heat exchanger, but the inline connector should be visible as the leads (grey to the inline connector/black from the inline connector run over the heat exchanger)

Measurement:

Test 1.
Measure resistance across the connections of the flying leads that runs to the flow switch.
Expected resistance open circuit.
Test 2.
Turn Hot Water Tap On.
Measure resistance across the connection of the flying lead that runs to the flow switch.
Expected resistance close circuit.