For the most part people are filling out their brackets qualitatively. Just an intuitive process where your brain strains out all the basketball you've watched and picks games. It's not a bad way to go. Remember, your 4th grade teacher always said to go with your first answer.
But if that isn't enough, there is a site out there that can add some quantitative substance to your decisions. I referenced a bunch of rankings and stats in the prior days and they all came from this site.
Pomroy brings Billy Beane's moneyball to the college basketball court. He has great stats that adjust offensive efficiency and defensive efficiency past just the superficial points allowed and scored. And by way of statistical alchemy creates his own formula for ranking all 344 teams, from Memphis to NJIT.
(Sidebar: my personal favorite stat is effective length which measures how each team's height is functional)
And for all those that are curious about what the stats have to say but don't wanna go to the site, Pomroy's best two teams are Memphis and North Carolina. In that order