they cant really tell who's going to be the best just looking at college hoops material and predraft workouts. what they can do is take an informed guess and if a team feels a person will not fit their plans for now or for later they will trade that person. E.g. Shaq. It was time for the lakers to rebuild and i think it was a pretty good trade and they will be a pretty good team in awhile but probably not win a championship.
GM's take chances and they have 2. Supreme example is Isiah Thomas. Who traded for eddy curry. personally i think it was a great trade and eddy curry will be the face of the franchise sooner or later. i also think if it doesnt work he gave up tyrus thomas who i dont think will be that great of a player anyway so does it really matter?