Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove one of the wires from the DHW microswitch

Measurement:

Test 1:
HW tap ON
Measure the resistance across the microswitch terminals.
Expect continuity / zero resistance.
Test 2:
HW Tap OFF
Expect open circuit / O.L.
Are the resistances measured as expected?
Note:
Image shown is behind servo valve for clarity but the test can be carried out with the valve still in place.