In response to the question in the OP, I think you may "know," with a faithful and prudential certainty, that you are called to a vocation of marriage, when you meet a girl/boy whom you desire to love for God's own sake, unto their salvation and yours; and because you discern a certain aspect of Godliness in them which you cannot help but cherish. For such a love is true charity, and charity cannot be wrong. The key is to let the pure love of God serve as the undercurrent and foundation of all your earthy loves. Any Catholic who finds himself "falling in love" with a girl, in the faith and for the sake of the faith, ought to feel confident that God has given His blessing to the marriage and intends it to take place. At that point, the only doubt that remains to him is the rather pedantic and hairspliting epistemological worry over "how do I know that this is real charity and not some passion of mine disguising itself as charity?" But he who asks that question is only dithering, hiding himself, holding something back; usually out of fear, or out of an ingrained habit of irony left over from the long years of waiting and disappointment. The matter is fairly easy to settle if you are honest with yourself, especially with the help of a kindly parent or priest. (One of the reasons that married life has suffered so horribly in recent years is that so many parents and priests are utterly without prudence themselves; and lacking this spiritual sight, they are unable to guide others in the right ways. Therefore, young people are left to find their own way to the mark; and if they manage to reach it at all -- an increasing rarity -- they do so late, and only after many mistakes.)
Remember, a vocation to marriage is not simply a call to marriage in general, but marriage to a particular spouse whom God has appointed for you. It would be absolutely contradictory to the goodness of God to suppose that He has destined you for marriage, but left you with no means of determining whom to marry or even whether to marry at all. When you meet her, you will know. Perhaps not right away, but you will know. Likewise if you have a vocation to the religious life. It is important to wait upon the Lord and let Him take charge of the whole proceedings; for then you can be sure not only that you ended up in the right vocation, but thay you ended up with the right spouse.
Unfortunately, the majority of married people do not wait upon the Lord. They choose a spouse to their own liking, based on some rather subjective and flimsy criteria, and go straight for it without having any idea what they're getting themselves into. The results of this strategy are on display in the form of ruined families, divorces, infidelities, illegitimacy, whole generations of dysfunctional children, and worldwide demographic collapse. You do not want to be affiliated with any of that. You want a happy and peaceful home, a strong and virtuous family, and descendents like the stars in the sky, all reigning as kings in heaven, as devoted servants of the Most High. Do you really think that God does not want to give you these good things? Of course He does! If you follow faithfully His commandments you will one day find your vocation all laid out for you like a table in the midst of your enemies. Then you will know that you are called to marriage; but more so to faithfulness and thanksgiving, to marvel eternally at the good things the Lord has done.