First of all, these guys are not sleepers. The "advanced" statistical community is well aware of all the players I am about to mention. I take no credit for discovering any of these guys. But I will take credit for understanding their potential values better than most NBA executives.
Especially in a draft that seems dubious up top, multiple later picks are the way to go. Cleveland (picks #24, #33, #34) for instance could probably draft all three of these guys I am about to talk about. If even one of the three turns out to be very good they would consider it a successful draft. That's the beauty of late picks - you only have to be right some of the time to make the whole thing worthwhile.
My process for evaluating players in the draft, and specifically with underrated players, is to look at the stats first. People like Ed Weiland, the Wages of Wins guys, and John Hollinger have all proven that statistical evaluation is extremely suitable for analyzing college prospects. I value this kind of input the most.
I also consider the eye test. Obviously it helps if a player looks good, and has a pro-style type of athleticism. Sometimes a player has only decent stats but looks amazing - I put considerable stock in that. But if a player looks good and has bad stats I will be leery of him.
Finally I think it is important to value the opinions of pundits on the ground, guys who speak to lots of pro scouts and see players work out firsthand, like Jonathan Givony and Chad Ford. Their opinions, which are usually similar to most NBA scouts, can be very valuable.
The point is you can't be too strict in how you analyze college players. There is a hell of a lot of uncertainty involved. To be a slave to stats when it comes to the draft is stupid. But to not deeply look at the numbers is even more foolish. If a little more merit was put on statistics it would make most teams draft better. This is why Weiland is probably the best at what he does - he comprehensively looks at the numbers and still understands they can hardly tell everything about a player.
But enough about this. Let's get on to three players who likely won't get picked until late in the first round or later. I expect all three to be very good professionals, and perhaps stars, if given the opportunity:
Jae Crowder, Marquette
Crowder is a nice example of an excellent prospect hiding in plain sight. He was Big East Player of the Year this past season, but somehow never generated much first round buzz. Most statheads love him. Crowder is one of those guys who does everything well, particularly defensively. The only real knock on him is his height - he's probably no taller than 6'5''. That said, I doubt he will have a hard time guarding forwards in the league. Defensively, Crowder looks to be an exceptional player. I fully expect him to to be able to defend multiple positions. On top of his excellent statistics, Crowder seems to possess the mentality to work hard on improving his game as his career progresses. Apparently in preparation for the next level he is already working on his shooting guard skills. He probably deserves to go in the lottery, and will instead likely drop to the middle of the second round. That makes him a rather obvious steal.
Will Barton, Memphis
Barton is another stathead gem. Memphis was kind of off the national radar this year, and in all likelihood that hurt Barton's draft stock. If he played at UNC or Duke he might be regarded as a lottery pick. He has that kind of talent. He can score, rebound, and pass at an above-average level - a skill set that should translate well to the NBA. If Barton can remain healthy (he's skinny) there is an excellent chance he will play in the league for a long time. Chad Ford the other day called Barton his favorite projected second rounder, and it's pretty easy to see why.
Draymond Green, Michigan State
Green is the kind of player who seems to always be picked later than he should be. An undersized power forward, he is an exceptional passer for his size and a good outside shooter. His numbers across the board are excellent. The concern at the next level is that he will not be able to defend forwards - a concern, like with Crowder, that holds little bearing. A creative player who comes with an excellent college pedigree, I expect Green to be a solid player in the league for the next decade.