He is in love with me. I can tell by the way he looks at me and how he reacts to me, besides him telling me so. But that doesn't mean he is my soul mate. What if he is my soul mate. What if he isn't.
I often think what if he is someone else' s soul mate, and I'm taking him away from her.
I note our story, how we met, note any signs that we might be soul mates, such as parallel events while we were separated or synchronicities. Pay attention and note any addictions or expectations.
Parallel events between him and an ex. They were young in a love / hate relationship, ended in hate, he took time to get over her, but that doesn't mean they aren't soul mates. When I see a big sign that indicates they might be soul mates, I feel sad that I may one day have to let him go.

He has a story with me that began when we were 15. He has a story with her that began after I left him, they triggered each other to the extreme. They're in the same career, (well they went to college together), even share the same interests. Sounds soulmatey, right? I had a story with another, which one day when I could feel my soul, I felt drawn away from him to never return, so he can't be my soul mate.
We were both engaged to our ex's in our 20's and the engagements ended near the same time, then we met again at a party. (Synchronistic events?) We separated again, then back together in our 30's.
Now for a while, he's been drawn to moving to a particular state on the west coast, and even more so of an urge to last month. I accidently found out, and he doesn't know it, but last month his ex has moved near that particular area he is drawn to. (Big sign?) No, I don't stalk her or try to keep up with her whereabouts. I just suddenly became curious about her. Weird.
I'm already holding onto him as if I will one day have to let him go. I may be wrong, but just going with the emotions, I guess.