When I feel complete freedom to be myself around her, she's got soul mate potential. Looks also matter, like, her face has to be nice and she has to be within reasonable health.
It always starts when both of us make solid eye contact and we have no problem picking up each others' verbal and nonverbal cues (which leads to an enjoyable several hours of conversation). There is also a mutual physical attraction that keeps us gravitating towards each other. It feels like a natural fit.
I'm a rigid and unorthodox guy and that helps me sort out the ones with potential and ones without. Some girls get that flabbergasted look on their face and take what I say the wrong way. Some feel intimidated, others feel offended. If they have any of those reactions, the potential is not there.
No I don't believe there is only one person for us.
Honestly, I think when we draw up ideas of how he / she should look, what qualities they should have etc we not only rarely wind up coming across them but more so we wind up overlooking them when they are actually presented to us. Reason being we are (1) lost and busy in our fantasy world to notice reality (2) fantasies are always highligted versions of reality - reality doesn't come with the 'halo'
When you meet your 'soul mate' you just know it's him / her, things just seem to fit into place all by itself as if it's on an autocomplete mode. you tend to like everything from their breath to the look in their eyes, words, heartbeat, actions, anger. You tend to find everything they do endearing, you want to fulfill every desire they have cause it fulfills you as a person seeing their smile.
I believe this, it's happened to me - things I'd even find disgusting with other people I'd find it endearing when it came from her :)
You just know. lol! Seriously there really isn't a way of telling...i guess when you found someone who makes you feel something that you never had or have not in a long time and you know you want to spend your life with them and don't need anyone else because they are everything and more that you ever wanted. I guess when it comes to that and it works out you know that is it.