(and you yourself are not Catholic
1. Not all Catholics are the same, and not all Catholics fully believe in and follow the doctrine and dogma of the Catholic Church. In other words, be sure to ask your prospective soulmate about their religious beliefs if you haven't already. It is vital to a marriage that both people accept each other's spirituality. If one wishes to convert the other, you need to back off now, and sit down and figure out _exactly_ what's going on in each other's heads. One partner thinking that the other is going to hell if they're not converted is not a good basis in a relationship.
2. You need to know your Catholic partner's exact stance on the entire contraception and natural family planning issue. The Catholic Church firmly believes that "In our society we have lost sight of the fundamental truth that if you are not ready for babies, you are not ready for sexual intercourse."* Personally, I have a big problem with this: first, who determines what "ready" is?(financially ready? mentally ready? Who defines mentally ready?), and what if some mature people know that they're not going to be mentally prepared for a child until they're 27 or 28? Should they delay marriage until then? How the hell are they going to forge a strong enough relationship that will prepare them for a child if they can't even live together?
3. Figure out everything you can possibly figure out about how you're going to raise the kids spiritually (if you're having kids at all) before you get married. The Catholic Church pretty much demands that the children of a Catholic be raised Catholic. This is a possibility for a sticky situation that should definitely be worked out before the relationship becomes permanent.
* Humane Vitae: A Challenge to Love, p. 10. Smith, Janet E. New Hope Publications