> In other words how can we be sure that it is our twin and not
> "soul mate" ?
Twin stories differ from a short, turbulent relationship,and a separation , to married couples who share a lifetime together. The bond they have is very telepathic, more so than any other relationship, it is also far more spiritual, but both have to be equally advanced. A good astrologer can see such a meeting reflected in the birth charts of both, because such a meeting is predestined. Even if the other half is not incarnate at this time, you and your twin are always in contact. If both are incarnate at the same time, they are drawn to each other like magnets.You've met before in past lives, and have shared experiences so there is a 'deja vu' feeling about it. You'll forgive easily,for there is a deeper understanding between the two of you.
Anyway, a soulmate relationship also serves to grow spiritually, and can be equally rewarding. In the end we are all part of one creation.