Outfielders: Knowing the Pitch

One of the advantages infielders have is the fact that they usually can know or find out what pitch is about to be thrown to the batter.  Attentive infielders look at every sign from the catcher to the pitcher in order to get a better idea where the batter may be more likely to hit the ball.  First basemen and third basemen probably won't be able to actually see the catcher's signs but with a little help from the shortstop and second baseman as described within this post HERE, they can easily find out.

Outfielders won't be able to see the sign but they
can see where the catcher sets up on the pitch.

Outfielders don't have this luxury.  They are too far away from the catcher to see the signs.  However, there are a couple things that can be done to help those guys get more information about the pitch. The first involves something middle infielders can do to help and the second involves something outfielders can do for themselves.
Some middle infielders, upon seeing the sign from the catcher, will flash a sign to the outfielders to indicate what pitch is going to be thrown.  The infielder starts by putting his bare hand behind his back in a fist.  If the catcher gives the sign for the fastball, the infielder leaves his hand in a fist.  If the sign is an off-speed pitch, the fielder opens his hand.  As players move up in levels, this option becomes more difficult because of the threat of the other team stealing the sign.  In college and pro ball, the bullpen is usually either down the line or beyond the outfield fence.  As a result, players on the other team can also see the hand behind the infielders' backs.  In this situation, some bullpen pitchers have worked out a system of relaying the pitch to the batter when they see what sign is flashed to the outfielders.  This is why you never see major league infielders do this for their outfielders.
The second option for outfielders is to just watch where the catcher sets up on the pitch.  Although they might not know the exact type of pitch that is being thrown, they still can get a feel for the location of the pitch in order to get a better idea of where the ball might be hit.
Regardless of how outfielders do it, it is important that they do not tip anything off too early.  A sharp offensive team will pay attention to what the outfielders do before the pitch.  If they take a step one way or the other before the pitch, they may be tipping off the type of pitch or location for the batters.
If it's done safely and correctly, outfielders can gain a lot by knowing the pitch or at least the location before the ball is hit.