With most bigs its a crap shoot. Most are inconsistant. They give you a 25 and 12 one game and a 7 and 6 the next. They'll have one foul in one game and foul out the next game. When you see one that puts up consistant numbers game after game then you probably have as close to a sure thing as your going to get.
The other problem you have in college is the inconsistant play of the guards. Their inability to get the ball to the big man inside. Aldrich doesn't have that problem. Kansas has good guard play, so any inconsistancy belongs to him. Same could be said for Cousins. So you can make fair judgements on them for the most part. The same can't be said for Alabi or Udoh, or Varanado. All three play on teams with guards that think their only job is to shoot three pointers.
Then you get a guy like Parakhouski who plays for Radford and puts up 25 and 12 every game. But he plays in a lesser conference against less talented competition. You can't fault his results, but you have to wonder how it translates to tougher competition. Thats why when you get a guy like Blake Griffin who put up 25 and 15 every game against the best competition in college, you know you have a special player. None of those around this year. But there will be a few that will turn out to be special. The trick is figuring out who they are.