I’m not really qualified to answer this question, since I am neither betrothed nor bound to anyone… but what the hell, I’m butting in anyway.
For a large portion of my adult life thus far, I never bought that whole “I just KNEW…” thing. I figured there had to be some other factors at work, here, and that if I asked those coupled folks who said they “just knew” the right questions, all would be (quite logically and scientifically) revealed.
I never found the right questions, however, because I rarely got more than, “I dunno, you just… FEEL it. You just KNOW.”
Bah. Humbug.
So there I am, happily doing my little bookstore job, happily (if coquettishly) pursuing one of my coworkers, on whom I had a little crush (and was sure the feeling was mutual, so it was just a matter of time)…
… and in walked The Knowledge. 
Said “knowledge” arrived in the form of a fella with whom I’d gone to school, but never really been friends… I basically just knew him well enough to know his first and last name (he, meanwhile, remembered the first thing I’d ever said to him in life… turns out he’d had a little crush on me back then)… but when he approached me, and I looked up into his face to utter the customary, chirpy, “Can I help you find something?”, there was like this little electric “BZZZZZZZZT!” that confused me and rendered me speechless for just a moment…
…and then… “I KNEW.”
(Schmaltzy side note: I even remember that my hand, seemingly of its own accord, reached up to shove his wind-blown hair out of his eye… I was utterly shocked by this (I am generally hyper-aware of physical boundaries, and only tend to go randomly touching people I know well enough to know where those boundaries are for them) and, at about mid-reach, froze in “What the hell am I DOING???” panic and frantically diverted the wayward hand to my OWN head, all the while still unable to say anything.)
My whole belief system was shattered at that moment, because suddenly I was one of those annoying people who simply shrug with that air of righteous Zen, and fail to give their inquisitors any valuable information as to the nature of their infatuation.
Suddenly I was one of those people who “couldn’t explain it.”
All I can say is that it was sort of like sinking into a nice warm bubble bath (if you enjoy that sort of thing, find it immensely relaxing and all), sort of like an instant release of all tension. Something inside you just sort of releases and goes, “Well, then. OK.”
And it was like having my own private Vulcan Mind Meld partner… Y’know, little things, like I’d get up in the morning and base my work-outfit choice on the “knowledge” that he’d be dropping by to visit me at work that day (without our having verbalized any plans to that effect). For part of the day, I might even keep an eye out for him, as I went about my work…
…then I’d usually convince myself that I was steadily scooting Around The Proverbial (Cuckoo!!) Bend, sitting here waiting for this boy based on some clairvoyant “hunch”… who was I, a Psychic Friend???
But, sure enough, just about at the point that I’d inwardly laughed the whole thing off, he’d show up, smiling and bearing confectionary treats for me. 
Stuff like that.
Now, of course, we’ve broken up… we were friends for awhile thereafter, but haven’t even been on speaking terms for three years… 
So much for “knowing”, eh? :rolleyes:
So now I figure that knowing who the right person is sort of depends upon a combination of logical considerations, like featherlou mentioned (compatibility, similar goals and values, etc.) and crotch sparks. 
I have to say, though, that I haven’t felt the same about anyone before or since that man (in that I haven’t felt that freaky little BZZT! or felt inexplicably compelled to push anyone’s hair out of his face on first sight), and one of the hardest things for me has been reconciling the way I felt (the “knowing”, if you will) about him and the fact that we didn’t end up together.
Still, ultimately I believe that things work out exactly as they’re supposed to, so maybe my crystal ball was just a little rusty at that time. Maybe Ms. Cleo was secretly siphoning my Psychic Power. 