I'll play nice.
As lame as it sounds, I just knew. I met my husband on a Saturday night. He called me the next Tuesday and we talked for more than three hours on the phone. By the end of that conversation, I just knew. We have extremely similar backgrounds, religious views, sense of humor and ideals. Our differences are also complementary (for the most part - our shared tendency towards procrastination isn't always the greatest, though). It didn't hurt that I thought he was really cute the night I met him, despite the shaved head and late 90s ensemble.
We've been together for 8.5 years and married for just over 5. We've had ups and downs, but for the most part it's been good.
ETA: plus, we make adorable, chubby, red-headed babies together.