A water damage sensor is a tiny transparent indicator inside or around the edge of your phone. Used mobile phones are checked to see if the indicator has turned pink. If it has, it's a sure-fire sign that the phone has been exposed to moisture.
Most newer Androids do not have built-in ambient temperature and humidity sensors. Some Androids that do have these sensors are the Samsung Galaxy Note 3, AGM X2, Galaxy S4, Motorola Moto X, and the Moto X 2nd Generation.