It can be very difficult indeed to know when you have found the one. For me from the very instant i met her, i felt like i had made a connection with her not in a sexual way... but in my heart. I guess you can only know in hindsight if she/he was the one. My advice is not to try and focus on finding the one... just focus on the current moment and hold the though of finding someone. Oh, if you are interested you can read myTake on how i met her.
You just know. There is no checklist, you just know it in your bones
For me, it takes at least a year to figure out if a woman is going to be compatible with me long-term. I don't believe in the concept of soulmates, just women who are compatible with me (and there are plenty of them out there). Read this:
I don't think you know if you found the one or not... I mean maybe I'm just bitter... but the girl I was with... I strongly felt like she was the one.. we were basically together for 6 months... and then she broke up with me 2 weeks ago Monday.. and it seems like she's already completely over me.