1: You have the first words your soulmate says to you somewhere on your body.

2: Anything you draw/write on your own skin appears on your soulmate's

3: You have a clock that counts down until you meet your soulmate.

4: You have half of a tattoo, and your soulmate has the other half.

5: You see only black and white until you meet your soulmate.

6: You share a dream with your soulmate when you're both asleep.

7: You have an hand print somewhere on your body where your soulmate will touch you when you first meet.

8: Everyone has a journal that allows them to write back and forth with their soulmate.

9: The classic red string of fate

(Whether the AU is set in Modern times or during the Taisho period, is up to you. And don't forget to pick one of these scenarios.)