One of the best sensors in smart phones is Proximity sensor. This helps to detect how close something to the phone screen and mostly to the ear. It is mostly placed above the screen near the area where camera is situated. It usually turns the screen off when the phone is placed around the ear during call. To check if it works dial 909 and call. Then cover any hole like area on the top of your phone. If the phone screen turns off, it works. If not restart your phone and dial again and cover that upper area to see.