The olive theory (and other "soulmate" tests)

I was rewatching HIMYM and was reminded of the Olive Theory (in that in every relationship, one person has to love olives, the other has to hate them). It is something that my friends and I joke about whenever one of us starts dating someone new... "But how do they feel about olives?"

Personally, I don't know if I buy into these "soulmate" tests. (IDK if I even buy into soulmates in general!) But I'm curious... what are other tests or theories when it comes to relationships that you've either had come up in your life, are from movies/tv, or you feel particularly superstitious about. (One of my best friends believes the olive theory so much that she stopped dating a guy bc he also loved them. Crazy!)