Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect brown leads from DHW microswitch

Measurement:

Test 1.
Measure resistance across the exposed terminals of the microswitch.
Expected resistance open circuit.

Test 2.
Turn Hot Water Tap On.
Measure resistance across the exposed terminals of the microswitch.
Expected resistance closed circuit.