It's impossible to know how an NFL player will fare during an upcoming season. There are projections based on stats and likelihoods, but even those are wrong sometimes. Injuries could derail a player's season, or he could just underperform.
That's part of what makes fantasy football drafts so difficult. No matter how much preparation you put into it, there's always a chance your players end up as busts and you end up at the bottom of your league's standings by the end of the season. And there may be nothing you can do about it.
The deepest position in the draft is wide receiver, as you can get potential sleepers in the late rounds.