That feeling that you "know" is not uncommon in early relationships. I think it's mostly infatuation or wishful thinking. I've "known" a few times. Time proved me wrong in every case.
A person's character reveals itself slowly, over time. When a relationship is new, both parties are on their best behavior, in carefully planned situations (dates), and most of what you know about the other person is what they tell you. IMO, it takes a lot of time, watching the person in day-to-day life, to really know them. How do they react to tough situations? Do they lose their temper? Blame others? Fold up and hide from the problem? Do they have your back, or do they react out of selfishness?
Are they the person you'd want at your side though illness, financial problems, raising a child, dealing with aging parents?
I think all you can know after a few weeks or months is that the relationship has potential. You can certainly know that you like the person, and that you're strongly attracted to them. Anything else is pure gambling. You WILL find out negative things about the person over months or years (we all have our not-so-great features). Whether those not-good features of their personality will be something you can live with, or something that's an absolute dealbreaker...that's a roll of the dice.
Marriage isn't something I'd want to roll the dice on.