quarterback reading the defense

Question
how should a quarterback read the defense in a passing situation? do you first look at how the safetys are aligned and how far the cornerbacks are off the line of scrimage? Should you look at a mismatch like a wide receiver on a corner? what are some the things to look for to determine where to pass and if the defense is blitzing.

Answer
    Kevin everything that you are saying is on the right track.  The key is being able to process this information, between the time it takes you to break the huddle and the snap of the ball.  Practice makes perfect, so it will take some time.  The QB needs to now what the defense looks like, so if your team audibles, you put your team into the right play for how the defense looks.  
    On a pass play, step one is to look and see how many safeties, there are.  This helps you to understand what coverage the defense is in.  (also watch tape of the other teams defense from the week before and study what coverage they usually use.)  If there is only one safety that usually means cover 3.(or 1/3's)  Two safeties usually means cover 2 or(2 deep zone)  Although both of these can also be man to man as well. After looking at the safeties then look at how close the cornerbacks are playing to the receiver.  If they are tight this usually means zone, with the safeties over the top as help.  If the corners are a little farther off this is usually man to man coverage.  It just takes time and dedication to learning and asking questions, to your coaches to learn coverage schemes.  
    Mismatches are an important thing for the QB to see.  If you have a 6'3 receiver going up against a 5'8 corner, look that receivers way especially on fade routes where your receiver can use his height and size advantage.  If your team has three wide receiver formations, I like to tell my qb's to look opposite of the twins side.  The reciever on the other side of the field will almost always be lock up in man coverage, with really no safty help, look to throw that way.  
    Two other key to teaching how to read the defense, is understanding pass progression, meaning your first receiver that you see open is the best read.  Don't over look the field, if you do this is when interceptions and sacks start to pile up.  Point two is to look for your receiver from inside to out.  Closest to qb to farthest away from you, especially if your team is a spread or multiple receiver offense.  This is not quit as important in standard I formation or pro set style teams.
    Last but not least is throwing the ball at the blitz.  Lets say outside backer is blitzing from your left the best place to throw the ball is to were the blitzing backer just vacated.  This is now the weak spot of the defense.  A quick slant would be a great call against that type of blitz.   
    There is alot to learn about playing qb, but it is fun to learn.  Hours and hours of practice against live defense is how you get better.  Hopefully these are a few tips that help you become a better player.  GOOD LUCK NEXT SEASON COACH NOEL!!