How do you know if they're "the one"?

About nine months ago, after meeting this guy at a party, I went on three dates with him. He was... amazing. He was everything I'd wanted and didn't know I wanted in a man, the sexual tension/chemistry between us was amazing (yes, I hooked up with him, though we didn't go "all the way"), and we had a mutual friend who vouched for him as a good person. I've dated before and not found a man like him.

Unfortunately, he decided that I wasn't "the one" for him and broke it off with me. Since then, I've gone on dates, met guys I liked, even hooked up with one. Still, occasionally, I return to thoughts about this guy and what a full life with him would be like. I know this is crazy, and I know that I'll never see him again.

I thought I would always know if I met "the one," but now I don't know. Clearly he's not "the one" or we'd be together. What "signs" or "criteria" do you use to know that the person you're with is the one for you? And for those of you who've found your "one," how long did you have to date them before you knew? Was it instant or did it take a while?
How do you know if they're "the one"?
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