Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove both yellow leads from water pressure switch

Measurement:

Measure resistance between each of the terminals of the water pressure switch, in turn, to a suitable earth point
Resistances should be greater than 1M Ohm on both tests.
Note:
Water pressure switch has three terminals, one of which is blocked off.