This could be done any way you want the most often used are
- matching tattoos/ birthmarks
- whatever you write on your body appears on them
- whatever scars you have they have
- whatever pain you feel they feel
- Hanahaki Disease until they reciprocate your feelings
- a mark on your body where they'll first touch you
- their first words to you tattooed on you
- everything is in black and white until you lock eyes with them, then everything in colour
- can telepathically hear each other's thoughts
- heterochromatic eyes where one of them is your colour and one of them is your soulmates
- a moving compass tattoo on your wrist which points in the direction of your soulmate
And if you can think of any others let me know
