It happened to me. I have only ever had one relationship before and that is with my husband. At the beginning of when we met I instantly knew that there was a connection on both sides though I didn't want to say so. But from that moment I knew that I loved him and he loved me and that this was a serious enough connection to get married. Since I have been with him, we have gone through problems but those problems being financial issues etc... We have never once got mad at each other even after confiding in each other things that could have potentially broke other relationships. We have each never lusted for anyone else. We have never argued, only in a joking way. We have always loved each other from the moment we met. So, yes, I did know we were going to live a long and happy life together in marriage.
You can be certain you love someone, sure. But to know you will end up marrying them will second guess every potential aspect of your relationship. It is romantic to look ahead; it is impossible to tell the future. I believe that you will both know you will end up being married with discussion.