For a quarterback to be good at assessing the defense, you must be a quick thinker and be able to make adjustments on the fly.
In order to do that (after the quarterback breaks the huddle) you will be walking toward the line of scrimmage, scanning your eyes from left to right. You should first make sure your offensive personnel are in the correct position to execute the play. From there look to the defensive in the middle part of the field. You want to see if they are in cover one, cover two, or cover three. Then you should check the position of the linebackers. Finally, scan back to the line of scrimmage to see how many defensive players are in the box.
Once you have done that assessment, you will determine if any adjustments are necessary to the play call and then get under center to snap the ball. A successful assessment is the start of a great play.