Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the microswitch from diverter valve body
Remove the wires from the microswitch noting their locations

Measurement:

Test 1:
Measure the resistance between C and NC - expect continuity / zero resistance.
Measure the resistance between C and NO - expect open circuit.
Test 2:
Gently press the switch of the microswitch and fix in the "pressed in position" using tape or a cable tie.
Measure the resistance between C and NC - expect open circuit.
Measure the resistance between C and NO - expect continuity/zero resistance.
Are the resistances measured as expected?
Note:
Terminals are labelled on the microswitch housing.