Learn how to read the opponent's backswing to know where he is going to hit his shot.
Look at Roger's back swing on this backhand groundstroke.
Detection of the shot is based entirely on the position of the racket head in the player's back swing when it changes direction from going back to going forward.
In this picture the racket head has passed behind Roger and is located over his back shoulder. This means he his about to hits a cross court backhand.