I'm not sure about "the one" as a concept I believe in, but I definitely believe it's possible to find a person you connect well enough with to stay with indefinitely.
For me, the first major sign was comfort. I was 18, insecure about myself and always felt awkward around men...I never quite felt pretty enough or like I was able to be 100% myself around them, I always felt like I had to put on some show or front, to keep them interested. With my Fiancé I instant felt totally comfortable being myself. It was like I'd known him my whole life after knowing him for 10 minutes. We just clicked and I felt accepted.
Next was our conversations - we could talk for hours and hours and hours about nothing. We still can.
I can spend 24/7 with him and not want to kill him. Sure we might get a bit snippy if we're together too much for too long, but I don't want to strangle him after 3 hours like I do most people. I am by nature a solitary person and have always liked my space so for me the fact that I am okay with him in my space all the time, is huge.
We just clicked so strongly and so instantly that I knew there had to be something extra special about our relationship from the very start