Unfortunately, there is absolutely no sure fire way of determining which QBs will make it big in the NFL and which will be busts. It has almost nothing to do with their physical talent and nothing to do with their production. The only real thing one can point to is "will this QB be able to learn to process the NFL speed of the game faster than it was in college". There's no real indication of who can "improve" their processing and who can't. If there was, teams would already be all over it. Kurt Warner, who may be the greatest undrafted QB in NFL history, says its simple: Every one of the prospects will need to improve to make it in the NFL. Predicting who can improve and who can't, is essentially impossible.
Every single guy in the draft has the potential to be either Tom Brady, or Jamarcus Russell. Anyone suggesting someone is a sure thing, is fooling themselves.
I suppose there's a little higher upside going with the physically talented players, but they will still need to improve their processing to achieve their upside. Then there's Brock Purdy or Tom Brady who are physically nothing special at all, but could process the NFL game at a high level. But they both slipped in the draft to the back end, as did Kurt Warner who slipped all the way through.
Adding to the answer, technique flaws can always be fixed, such as footwork. But processing limits, the ability to operate under increased pressure, is typically a "you have it or you don't" skill. It can develop a little, but it has to develop a LOT from college to the pro's. So the only real deal breaker, is the one that cannot be seen or predicted.
