This question divides by zero.
How do you know someone is the person you want to marry? You date them. For a long time, in fact, and even then there's no guarantee you're going to be right (a year and a half of dating and half a year of living together before we got married didn't stop my ex-wife and I from divorcing five years later).
Now then, example time:
There's a girl I go to school with. Accounting major, good student, good looks, and if I were 21 and not 33 I might think "zomg, she's wife material, I better date her." But being older and more mature I at least asked her out on a date first. To say the date went "poorly" is putting it mildly. She's a vegan PeTA freak whose views politically are somewhere to the left of Karl Marx, while I'm one of Denis Leary's "angry, gun-toting, meat-eating fucking people." Preconceived notion? Busted.
On the other hand I once went on a date that was more out of her friend telling me "she's really been lonely lately, someone should keep her company" (literally a pity date, although I didn't tell her that)...and had a whirlwind six-month romance that only ended when she pushed me too hard for a commitment I wasn't ready for. Her trying to be "wife material" drove me away.
So you tell me. Is it not an exercise in futility to try and determine someone's marriageability before you date them for awhile first?