Anyone who tells you that there is a full proof way to evaluate QBs to find your franchise QBs from your busts is talking crazy. At the end of the day, there's no full proof way to evaluate QBs especially when the most important aspect (what is between the ears) isn't something that we as arm-chair GMs are privy to. Many years ago, Bill Parcells had a list of "rules" he used when evaluating QBs, and it showed some promise.
* He must be a senior
* He must be a college graduate
* He must be a 3 year starter
* He must win at least 23 games
All of these points of emphasis for Parcells are based on merit, although some might be viewed as archaic. The college senior rule is probably the most outdated, especially when most legitimate QB prospects often declare after their junior year. If you implemented this rule in this most previous draft class, your top QB would likely have been one of Davis Webb, Nathan Peterman, or Josh Dobbs. Not exactly a group that inspires confidence. Since the 2011 draft, only one senior QB has been drafted in the first round which was E.J. Manuel. If you eliminate any underclassmen QB, you're eliminating a large portion of QBs who are entering the draft. This is likely something that would likely be scrapped given the changes the NFL has gone where underclassmen declaring is almost expected especially if they have the buzz behind them. The second rule, the college graduate rule, likely ties into the previous rule. Again, you can probably lump this into the first rule, but given the change in climate where it is expected that underclassmen QB will declare if their stock is sufficiently high.
To a lesser extent, the 3rd rule also ties into the first two. While the expectation in early years was that a QB stayed in school throughout their entire career, it is no longer expected for college QBs to be a 3 year starter unless they're a starting QB as a freshman. At this point, two years is expected to be timeline that you will see an QB prospect as a starting QB. With a shorter than expected time frame for your QB prospects to be in college, you have less games to evaluate them in.
And the final rule is tied into that 3rd rule. In a best case scenario, in two seasons you'll get a chance to watch the QB play in 30 games which gives you ample time to evaluate the QB. That's 12 regular season games, a championship game, and potentially a bowl games to evaluate them in. If they don't make the conference championship game, you still get 24 games assuming he can stay healthy over his two seasons as a starting QB. In that scenario, you're looking at an a winning percentage of 77%, which is quite high. Without the added playoff wins, you're looking at a winning percentage of 88% which is even higher and probably more unrealistic. Odds are the number is probably closer to 75%, maybe even a bit lower.
All in all, Bill Parcells had a set of rules that he believed to be his rules. But like all rules, there are exceptions to them. These rules help mitigate those risks. Parcells is probably less willing to gamble at that position, but probably also more likely to avoid busts.