I'm sure you all know this one, in my opinion, its one of the classics, but I'm going to put it in here anyways. I don't remember where I found it, I made it up for all I know, but it's been in my head for so long, I don't remember.
The soulmates each have a tattoo that symbolizes the other person in some way. As they get closer to each other, there are plenty of different scenarios we could take. It changes temperature, fades away, changes, etc.