We'll there are certain sites out there where you can get training camp reports, I suggest taking a look at those. Although they are mostly pay sites though I believe.
You may also want to look at players who either A) Are exceptionally high rated at an aspect of their position...such as being very fast, being tall, having good hands are some aspects you might look for in a receiver. and B) Look at guys lower on the depth chart who either Compliment the players higher on the depth chart, or are able to do something the players above him can't...or at least do it better than them significantly enough that they would get chances to shine.
And then theres the lucky situation factor..although this kind of is a little less about training camp. Pay close attention to players on the depth chart behind other players that have any doubts, whether it be injury, contract dispute, or any number of other reasons that they may be doubted.
Now I know I mentioned injury in the past paragraph, but it deserves its own paragraph. Injuries happen. Keep a close eye on any player who is behind a starter on a depth chart who may have injury concerns, whether it be something small going into training camp, suffered at training camp, anything where the miss time or there is speculation about there being an Issue...an example is Clinton Portis, suffered a shoulder injury in pre-season, and eventually ended his season early. And in the same sense, take a look at players deep on teams that run RBBC, because if one of the players goes down, or is traded or anything, its likely another player will step up and play..ala projected DeDe Dorsey
so basically its not all about ESP...but knowing what to look for, and situations that they could excel into.