iPhone SDK - get/calculate camera field of view (FOV)

I'm working on a simple augmented开发者_运维技巧 reality application with camera overlay in detecting nearby stores. Anyways, I have three latitude/longitude points 'A','B' and 'C' using core data. Point 'A' is the users current location. Point 'B' and 'C' represent fixed store locations.

1. How can i determine the position the user is facing?.
2. How do i know if points 'A' has either of 'B' or 'C' in its line of sight?