I hate to repeat what I've seen above but 'I just knew'.
I'd had a few serious relationships before DH, even as I was in them I knew I wasn't going to be with them forever.
I knew DH about 3 years before we got together. We knew there was chemistry but we'd both had partners and I was quite a bit younger.
He bought a house with his GF and moved away about 50 miles. I was heartbroken. I thought - this is it. I've lost my chance, they're going to have a baby and get married etc. I barely saw him for 9 months. He phoned me one day upset saying she thought she was pregnant. 
A few days later he called to say she wasn't pregnant. I couldn't tell if he was relieved and didn't want to tell him how I felt as I thought he was happy, he'd never had a serious relationship before and I really wanted him to be happy.
Another month passed and I get a call from him. He'd moved out but they were still together. We met up as friends for a chat as he was back living in town. He told me how he loved and cared for her but he just knew that she wasn't the one. He had been unhappy for a while and had gone home that evening, told her he was sorry, packed a bag and left. She had known this was coming and felt it too. I had broken up with my boyfriend of 2 years a few months before but hadn't told him.
This was it. I broke down, I told him I loved him, I had loved him for a long time and that life was too short. I knew he was the one and I'd have told him earlier but I thought he was happy. He kissed me and that was it. I told him we would see each other as friends and take things very slow as him and GF were selling house etc and wanted to give him some time and space. But when we were together it was just heaven, the feeling of electricity and butterflies combined around him.
We moved in together 9 months later, he proposed 6 months after, we got married a year after that and 5 months later I was pregnant with DS. We have been married 5 years now and I am still sooo happy. That passion is still there, I still look at him and get that feeling. I am so lucky to have him.