(1) Interview them carefully as regards lifestyle and have them pledge and sign a document stating that they are not homosexual and have no designs on boys.
(2) Instruct the boys that anytime a scoutmaster or leader acts sexually toward them, in the slightest fashion, they are to report it immediately.
(3) Have each scoutmaster agree by contract, in advance, to the civil penalties they will incur (e.g., big-time $$) in addition to criminal prosecution should they so much as fondle a boy.
While well-meaning, and while #2 especially is very important (and the BSA has a video called "A Time To Tell" that instructs this), this could have been prevented if the adults involved would just stand up to their obligations.
Every unit has a sponsor (VFW, Lions Club, American Legion Post, church, etc.). The sponsor is supposed to interview and evaluate every single person that volunteers for leadership. While that might not have caught this guy, it would catch some.
The sponsor is also supposed to appoint a unit committee. The unit committee's job, among other things, is to supervise the adult leaders (Scoutmaster, etc.) and make sure that they are a) doing their jobs, and b) following BSA policies. That means that the members of the unit committee, usually parents of Scouts and/or ex-leaders whose son's have aged out of the program, should be going on unit outings and should be questioning both the Scouts (during Boards of Review) and leaders (during unit committee meetings) to see if everything's on the up and up.
They should be making sure that Youth Protection policies are being followed. That would have stopped this, since both Youth Protection and Two-Deep Leadership principles require that two leaders be present at all unit functions at all times (unless you have multiple leaders colluding in such activities, which is quite rare).
The committee should also be making sure that parents show up at activities to participate. The BSA doesn't exist to give you a chance to let someone else raise your kid. Both Scouts and their parents are supposed to participate in Scouting. And occurrences such as this is one of the reasons why.
My point being that there are structures and policies that, if followed, are quite reliable in preventing such evil things. But it seems that occasionally, there are parents who can't be bothered to learn about and follow them.