With current pro offenses and coaches more willing to change their offenses to better suit their quarterbacks, it's actually much easier in recent years to predict pro success.
At least right now, gone are the days of, "Well, he wasn't that great in college, but his skill set will translate to the pros." The best current predictor for pro success is college success.
A QB's really good in college? Cool, he's now got a really good chance to be a pro. Don't overthink it---If your pick comes up and there's a really good college QB available, hell, why not?