There are simply some people in your life that you can never forget. Not as in forgotten forget, but people that would always revisit your mind over and over again. For many others, the phrase "out of sight, out of mind" applies.
These special people are the kind that no matter how long ago you have last seen them, the latest, like all other memories of them, never seem to wane. Even, after many days parting you and that person, memories shared do not come rushing into you mind. Simply because they have never left your mind.
I guess this if the defining factor. You would know if its true love if that person sticks like super-glue in your mind after a long long time of separation without fading one inch and not only that, he/she is still the same long after.