I didn't invent this, but thought I'd pass it along. Sorry that I don't have any diagrams, those would help. From what I remember a lot of this came from Lindy Infante, but it's been used by lots of great passing coaches for a long time. This deescription is close to what I've always taught:
POST-SNAP READS (“READING THE SQUARE”):
The most important area for determining secondary coverages is the middle of the field about 15 to 25 yards deep and about 2 yards inside of each hash. We call this area the “square”.
We normally read the “square” in our drop back passing game. Reading the “square” becomes necessary when it is impossible to determine what the coverage they are in before the snap or to make sure of secondary coverage after the snap.
In reading the “square” the QB simply looks down the middle of the field. He should not focus on either Safety but see them both in his peripheral vision.
A) If neither Safety shows up in the “square”, and both are deep, it will indicate a form of Cover 2. A quick check of Corner alignment and play will indicate whether it is a 2/Man or 2/Zone. If neither Safety shows up in the “square” and both are shallow, it will indicate a Cover 0 (blitz look).
B) If the Strong Safety shows up in the “square”, this will indicate a Cover 3 rolled weak or possibly a Cover 1.
C) If the Weak Safety shows up in the “square”, this will indicate a strong side coverage. It could be a Cover 3 or a Cover 1. If the coverage is Cover 3, it could be a Cover 3/Sky (Safety), or a Cover 3/Cloud (Corner), depending on who has the short zone.
NOTE: When either of the Safeties shows up in the “square”, the best percentage area to throw the ball in is the side that he came from! If NEITHER of the Safeties show up in the “square” – throwing the ball into the “square” is a high percentage throw.