How do you know you've met THE ONE?

Been dating a guy for slightly over 2 months. We are both the type that wants to take things slow. Yet in our conversations lately, we've admitted things have moved faster than either anticipated. no complaining on either party. Every once in a while, we'll say to each other how happy we are to have met and believe we'll be together for a long time. in his words: "I know we'll be around 3, 4, 5 years from now." a part of me feels that is our way of safely talking about a possibility of marriage down the road. without freaking the other one out, since we are still learning about each other and only met 2 months ago. However, I have heard stories about people getting married within 3 or 4 months of meeting. so I am curious of all you married folks, how and when did you know you were with the one you would marry?