How do you find out whether the other person is the right one?

I think it's more important to recognize that everyone and everything changes with time. you may find the right one for right now, but you or the right one may change. so you either have to find someone who's always going to be flexible enough to change with you (or at least accept your changes, and the same applies in the opposite direction), or you're going to have to find someone you're willing to be so devoted to that you'll stay with them through any and all changes (and ideally the opposite applies, again), or you just have to accept that life is chaos that can never be in check and enjoy who you're with while you can and always be ready to let go and move along.

none of those seem like very good answers. sorry.