Different Types of Rain Sensor
See a Plastic cover around your Mirror? You probably have a Rain Sensor, Forward viewing Camera or Humidity Sensor.
Rain Sensors are easy enough to Spot, See Below for some Examples.
How Does a Rain Sensor Work?
The Basic principle involved in Rain Sensor Operation is light refraction. When small lasers are bounced off the surface of the glass, to small light sensitive recievers, any water on the windshield will allow a small fraction of that laser to refract. When the Windshield is dry, the laser bounces perfectly, and no light is lost. The more Water there is on the surface of the windshield, the more light refracts, and the sensor knows to speed up the wipers.
Rain Sensing Wiper functions usually include an auto setting, much like the "intermittent" setting, which allows the user to determine just how sensitive they want their Rain sensor to be under certain conditions. The figure below is helpful in depicting how a rain sensor in action works.