Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect plug from water pressure switch
Make a note of its orientation as it is possible to refit incorrectly

Measurement:

Measure the resistance across the two
spades on the water pressure switch.
Note:
Water pressure switch may differ in appearance
but test remains the same.