To me, opportunity is the best measure for potential sleeper success. Young guys like rookies or 2nd year players who maybe weren't quite ready the pervious season or dealt with injuries, but who are gifted athletes are the types of guys I looks at. But opportunity is IMO critical to identifying potential sleepers.

For wideouts, I look for a solid, proven QB. I also look for an uncertain depth chart at the WR position. If there is a superstar there, chances are you aren't going to get a massive breakout season from a young guy, barring an injury to that superstar. Also, the WR2 and WR3 positions should not be filled with proven, solid players. You want to have a guy that can win the starting job, even if they are no predicted to. I look at Perriman and Parker as this year's rookie class' best options to break out because they both have solid QBs throwing to them, are in proven passing offenses, and can both potentially win the WR1 spot on the team if they perform well.

For RBs, I look for a few different things. Instead of a QB, I'm looking for a solid offensive line, a good defense (to keep scores low and opposing offenses off the field), and uncertainty on the depth chart. However, at this position, injuries happen often, so the depth chart isn't as important as with WRs. In that case, I looks for starting RBs who maybe are decent players, but whose production could pretty easily be replaced. Ameer Abdullah is one such back, with Joique Bell the projected starter. Other young backs I like include Duke Johnson, who has an excellent OL and a decent defense, and a less than certain depth chart. I also like David Johnson, who is behind an injury prone Ellington. The Cards have a very good defense, and improved OL, and good enough WRs to draw coverage away from the RB.

All of these guys have opportunity, and are in decent positions to succeed for various reasons. So basically, identify opportunity, figure out who is there to take advantage of it, then decide who is the most talented player in the best position to succeed. While the rookies that I listed here aren't exactly sleepers for the most part, they do help to illustrate what to look for.