Don't bear the yoke with unbelievers. But beyond that, make sure your spouse is a solid, serious Catholic. That shared faith will give you the means to solve any problems and challenges that will appear later in your married life.
Ladies: make sure your prospective spouse can work. Make sure he can do things he "doesn't want to do" -- in other words, make sure he has a certain minimum level of mortification or self-discipline. Without that, he will be nothing but trouble. In fact, without self-discipline he's likely to be wallowing in sin (rather than fighting sin at all times) as well.
Men: The above advice applies to ladies as well -- they need to be able to work, albeit different kinds of work (housework, being pregnant, raising children, homeschooling, "frugality" tasks to help save the household money, etc.) Make sure your wife has a low degree of feminism. 0% is too much to hope for, but make sure she's content being a stay-at-home wife and mother rather than the co-equal head of the household. Make sure she's willing to believe that motherhood is a lofty role and not some kind of degrading role, beneath her dignity, invented by "the Patriarchy". Make sure she hasn't bought into the bill of goods that a career (working for The Man in a cubicle 40 hours a week) is the only way to live a fulfilling life.