It's different for everyone, I can only tell you how I knew. I knew the moment we met, which sounds cliche but we met online. We know each other better than we know ourselves. I knew he was the one, my soul mate, because there was no doubt in my mind. There was no little voice in the back of my head going "what if he's not the one?" Most of the time, you just know. I tell people, "You'll just know, but if you have to ask if it's love, then it's not."