This falls into the intagibles/team game stuff but you can measure it as well: a QBs college record.
Look at the list of all the QBs who have been to the SB in the last 10 years.
All of them were winners in college - and almost all 10+ wins their final season. Teams and the QB himself have to believe they can win all the time. I think everyone needs to read the Evaluating the Draft post http://www.windycitygridiron.com/2017/2/1/14470472/evaluating-the-draft . It lays out Bill Walsh's thinking on the draft. You see him touch on a lot of the measurable items but he consistently points to leadership. This needs to be a top concern of teams drafting a QB.
I'm sorry but if a guy doesn't inspire his team on the field in college, no number of measurable traits will work. It's the NFL, everyone has elite measurable traits. But teams need to believe they're QB will help them win. And the QB has to believe they are a winner and have won games.
I'd be willing to bet the Bears players would be a hell of a lot more intrigued if they brought Watson to the team rather than Kizer, Trubisky, Mahomes or Webb.