Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Remove the White spaded wire and the Black spaded wire from the Reset Switch and measure the Resistance across the Switch.
With the Reset Switch at rest the Resistance across it should be short circuit, when the Switch is depressed the Resistance should be open circuit.
Reconnect wires after test.
With the Reset Switch at rest the Resistance across it should be short circuit, when the Switch is depressed the Resistance should be open circuit.
Reconnect wires after test.
Isolate as per TOPs P4
and refit as necessary