Except we keep learning that it isn't that easy to spot the "true high-high-end talent." For instance, I am in shock B.J. Upton isn't a stud player at this very moment. Seriously, he had everything -- five tools, great instincts, loved the game. At age 18 he seemed like he was only a heartbeat away from the majors. Maybe he'll still get it together to be a good player, but he looked like Cooperstown was in his future five years ago.
I'd say it is relatively easy. That's why most of the kids drafted in the early rounds are on everybody's radar. No one's saying there is such thing as a sure thing (you mention Upton -- the college pitcher taken before him, Bullington, is pretty assuredly a bust, while BJ has plenty of time to become a good player, starting this year when he'll get regular ABs in the majors). My position is that you shouldn't rule out the HS crop at any point in the draft -- especially what appears to be the very high-end talent there. High-school athletes are bigger and stronger than they've ever been and the parental focus on getting them trained to perform at the next level, whatever that may be, is white hot these days. It extends from personal trainers to practically year-round high-level competition. Kids who are excelling against the best competition in that environment, who still have room to mature physically, are better bets than they've ever been, in my opinion.