It knows because the proximity sensors are located next to the ear speaker. the microphone, on the other hand, is on the bottom. So the only way you could "fool" the proximity sensors with your mouth would be to hold the earpiece to your lips, but then you wouldn't be talking toward the microphone, and the noise canceling wouldn't work well because it's built to work best when the phone is held against your head. Nobody is going to try to talk into the earpiece.
10/19/11 at 6:31pm