I think it is a feeling you have when you are with that person, a feeling that you are one. It is hard to explain. I think you will know when you find it. And if I can recommend anything, don't be afraid if you find this with someone you just met. I felt this with a man I met and lost him because I got afraid and now I don't know if I will ever see him again.
So, it could happen to you any where and any second. Hope you find your soulmate.