Q1: like blade said, with a c_trace in the direction the player is facing (for optimising speed you could first check if the player is turned at your direction at all, and if so, perform a c_trace to see if there are any objects interfering the player's sight).

Q2: Depends what you mean with 'exactly behind'. Does distance matter? Can objects stand between you and the player? And practically spoken, you can never be exactly behind a player. You will need a definition of what is 'behind'. E.g. define "between 170 and 190 degrees away from player's facing-direction, and within 50 quants" as "behind the player".