Be sure to say the rosary together daily, if possible. Together is important.

Carefully examine whether one of you may have a vocation to the Church before getting too serious.

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.

Make sure that he/she agrees to or is at least open to the idea  of homeschooling your prospective children, as it could be that no other option will be available to Catholics. 
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And while you are at it, consider the other's attitude to health issues such as whether or not to vaccinate.

Consider the possibility that you can not have children since it is not a guarantee.  Discuss this possibility with your potential spouse.  Given our older age when we met, we were already on board with "if it's God's Will", so this wasn't an issue for us, but I imagine that younger Catholic couples don't even consider the possibility of infertility.

And equally if you marry young that you may be blessed with a very large family.  

I’ve always wondered what happens if a husband or wife realizes to Late they were called to priesthood/nun life? That couldn’t be a very good feeling.

The Byzantine/(also the Orthodox Church) has married subdeacons/deacons, but that system hasn't been in place in the Western Church for thousands of years.

So, I don't know. That's actually a very good question.

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.

Matthew, I think this is great. What feminist don’t understand is that while motherhood is rewarding. It takes time to see that reward. Every milestone of religious growth, mental, and physical is a compliment to the mother’s perseverance. It took me a while to understand that the best reward takes time.