Set Appliance State:

On/Off Switch OFF
Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove one of the wires from the boiler On/Off switch

Measurement:

Test 1.
Measure the resistance across the terminals of the boiler On/Off switch. Resistance should be open circuit.
Boiler On/Off Switch On.
Test 2.
Repeat test. Resistance should be closed circuit.