Everyone 'knows' until they stop knowing/get the ick/get bored/realise he has a pregnant girlfriend/figure out that while he's great, he's moving to Bogota in eight weeks etc etc.
And you'll get a disproportionate number of (currently) happy endings on this thread, whereas in fact some kind of mass survey would pull up huge numbers of people who knew they'd met 'the one', but are no longer in that relationship for various reasons. Very few people marry without feeling sure and happy in their choice.
Yes, it's perfectly possible you have met someone you can happily spend your life with and I hope so! but it's also perfectly possibly you're in a blissful oxytocin and dopamine-fuelled fug and can't yet judge.
I was thinking about this recently because I came across detailed diaries of the beginning of a long-past relationship. I am not a gullible or inexperienced person, and I was absolutely certain I had met the love of my life. We never stopped talking, the sex was wonderful, he was attractive, clever, kind, endlessly interesting etc etc. It was like recognising someone I'd loved in another life. Yadda yadda. But actually, after the initial glow dimmed slightly a year in, I realised he was a fundamentally weak, passive man I'd projected my own sparkle onto. All his significant relationships had been with clever, successful, lively women who had initiated things and eventually realised that, outside the context of their relationship, he was a man with no interests, friends, passions -- he borrowed them all from whoever he was with.
I am genuinely absolutely horrified by those diaries now. I was so fundamentally wrong about him.