For each and every person, finding their soulmate is a completely different way. Some people share the same types of soulmate 'keys' and whatnot, but that doesn't mean they're soulmates.
Examples of different types of keys:
-If you can see wings on a person and they can see yours, that means you're soulmates.
-You're born with only being able to see the colors black and white in the world around you. If you touch a person and suddenly color washes over everything, and the same thing happens to that person, you're soulmates.
-The first words you say to your soulmate is tattooed on a person's wrist. Same for the other person. That means you're soulmates.
-Your soulmate's name is tattooed on your wrist, and your name is tattooed on your soulmate's wrist.