Listening for the kick is what referees do too. When I am on the line as an AR, I always look for the offsides and listen to the ball. If you watch the ball get kicked, then turn your head, you are going to make the wrong call by assuming that the player was offsides. The AR's should be be even to the second to last defender, watching straight across the field, and should hear the kick happen. That is the way you are taught, to do it any differently is to do a disservice to all the players on the field.
