Was just going to post the same thing. Baseball Prospectus, Baseball America, and Keith Law over at ESPN all have scouting contacts with every organization. They talk to them all the time. Along with that, they do plenty of scouting themselves. Kevin Goldstein said he would absolutely love to be employed as a major league scout but for whatever reason that hasn't happened yet.
The important thing to note, is that team scouts may have players ranked higher or lower than where those players are actually picked. Sometimes teams will try to save money in the draft, by refusing to spend the money they are alotted. Or in the case with the way the draft used to be and still is to some extent, some teams would never sign guys above slot.
That being said, yes, players are generally seperated into different tiers. Also, among BA, BP, and Law they can all disagree quite a bit about where to rank some players. You're trying to predict the future with these guys, and the best thing you can do is look at their tools, their makeup and try to estimate where they'll end up.