The two keys to smarts for a SAF is reading offenses (no easy task) and doing it at game speed.  I have a lot of respect for safeties.  I was defensive coordiantor at the high scool level, and I doubt I could break down an offense in less than ten to fifteen seconds.  What I did on paper and chalk boards these guys do in seconds, and they're out of breath, AND they do it on a pro level (light years more complicated than anything I ever dealt with).

I don't know how pro level deep zone coverage safeties are taught to observe the offense.  My direction to safeties was primarily to keep "the entire play in front of you".  This means no WR or TE gets behind you.  But you can't "coach smarts".  A good pair of SAFs know that one guy (the other SAF) might assist a CB covering a WR streaking deep, while the other one (himself) reads a WR breaking off across the field in a slant.  They get to know each other and where each guy will go without coaching.  There are too many offensive variations to have a set and easy rule for where to have SAFs go in their zones, so you trust them to be smart and to work together not to get in each others way.  They learn mostly from watching film and seeing where they can improve, or watching film of other safeties and pointing out good and bad decisions.  We had a few plays drawn up (man, double coverage, other areas to zone) for odd situations (never a blitz in my system), but most of what they did was learned and not taught if you understand what I mean.

As far as where their eyes are, at the start of the play they watch any receiver eligible players (including the RB and FB).  It is to be hoped that the SAF gets an idea from what he sees what the play may be.  We drilled our SAFs on game film to read rush versus pass by how offensive players lined up (how a receiver might stance for each play without realizing it for instance).  The SAFs then take a brief look at the o-lines feet for more clues.  After this they watch the QB and the ball, watching all the eligible receivers with their periphreal vision.  Once the play is beyond rush the SAFs pick seperate targets presenting the biggest shot at a blown or deep play and take him.

Again, the SAFs read each other for who to take.  Short passes are the most difficult, since the SAFs have a harder time picking a target (we called the target for safeties a "principal").  Rarely the SAFs keep their zone until the last minute, but a good SAF will read the play as it erupts and start taking their angle to prevent the big play.  An excellent SAF is in position before the balls arrives to make the hit.  I didn't want my SAFs going for the INT because a blown attempt can allow the WR to get downfield, but I told the kids they'd never get balled out if they got it.  If they missed it they caught hell no matter how well the play turned out (from the d-backs coach).  Intelligence was the first thing I looked for, then speed.  If a kid couldn't tackle they never got a look for DEF.  So our emphasis was that the safety is like a goalie.  His job isn't to make spectacular turnovers, it is to stop TDs.

Bear in mind on this one, the pros are different from high school.  That's the best I can do.  The safeties in my experience might be a little more complicated than my description if I went into further detail, but I obviously can only begin to try to describe the pro versions.

Hope that gets you started at least, even if it doesn't answer the question as well as a pro coach.