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 its housing and disconnect wires from pump proving microswitch

Measurement:

Make the microswitch by pushing in the button while measuring the resistance between the common and normally open terminals of the microswitch.
Resistance should be short circuit.
(zero resistance measured)