Where is the sensor...

that detects if the phone is at your ear or not? It's really annoying when it fails, and after I take the phone away from my ear it refuses to turn the screen on so I can, say, hang up (without using the top button), or show me who I'm calling. So I figure the best way to fix this is to bring the sensor towards and away from my ear in the hopes that at some point it'll pick up that I want the screen, but in order to do that I need to know where the sensor is...thanks!