I'm not trying to be funny here but I think you are over complicating things a little bit. They do what you said they just use a little intuition to decide if the kid is a good character fit and comes from a good home. Wint and Riep probably even passed the "test". Its inexcusable and disgusting if true, and they deserve to be locked up for a very long time. The action was selfish and immature, and that's not an excuse for it, that's an indictment of the young men who perpetrated the crime. Theres no evidence to suggest at this point that blame should be pointed to their parents, mentors, or anyone but Amir and Jahsen.
Long winded of saying that I don't think there is some real test or anything. These kids just likely came from good or decent homes and seemed to pass the character test. They failed everyone else around them.