Where can I find the temp sensor?

I have a '95 Mercury Sable 3.0L 6cyl. I recently drove my friend somewhere on a hot day, had the ac on and the car overheated and shut down (vapor lock I think), but the temp gauge read cold. The car still runs great and has never overheated before or after that time. Where would the sensor be located and how can I test to see what is wrong? Also, no check engine light is present.