Generally speaking, you will feel a sense of familiarity between and ease between each other. In the case of my best friend, she thought she knew me from somewhere. I thought the same. We'd never met, but it's always been easy to talk and hang out with each other. There seems to be this inner knowing that we've been in each other's lives before we met actually met. I also experience quite a bit of deja vu with people from past lives.