Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove 1 connector from the summer/winter switch (yellow leads)
Summer/winter switch to WINTER position

Measurement:

Measure the resistance across the summer/winter switch, (middle connector with yellow leads).
Expect continuity
Note:
Some models incorporate a rocker switch and some are a push switch. They operate in the same manner.