Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the red leads from the pump proving microswitch
Remove the microswitch assembly from the boiler

Measurement:

Test: 1
Measure the resistance across the microswitch
Expect open circuit.
Test 2:
Depress the micro switch with your finger to simulate the
flow switch spindle.
Measure the resistance across the microswitch
Expect continuity / zero resistance.