Raid the wife/mothers kitchen, (don't know your age so I don't know whop's kitchen it is) find a thermometer that will go to +250 degrees. Get a pan of water and place the switch in it, ONLY the part that is usually in coolant, not the whole thing. Place an Ohm meter across the two leads and measure resistance, turn on the burner and watch the tempature. The switch should close around 210 degrees.
Thats the book procedure.
Another option is to connect an volt meter or light bulb, (12 volt) on the fan side of the switch, other side to frame or battery negative, start the motor and wait till the high temp light comes on, the fan should start before the light is on.