I teach my Quarterback's to read the "Press Line" and then see their eyes. Each week we decide what our "press line" is, typically about 8 yards from the line of scrimmage. By counting the player deeper than that line, you can determine a basic of what the defense is trying to do. Secondly, you need to see the defenders eyes, if they are looking at you it a Zone coverage, if they are looking at the WR its man to man.
Check out the article i wrote for CoachUp on identifying defensive Alignments: