Why does it matter? There is a belief you are reborn surrounded by similar people, so your child might be your grandmother in a past life for instance, but it's not proven. Some people have also been known to write into their handfasting vows to find each other in the next reincarnation. This all being said, that was your past life, a completely different person, while past lives can be interesting, it was the past. This person might of been your killer in a previous lifetime, but in this one they might be your lover. Each life is a new experience.
I do not know if there is a spell, but you could both try a past life regression and compare if either of you were in a past life, but that's a shot in the dark as far as how well the other person is at regression and if you see the same lifetime.