Hm, this is a hard thing to describe. I guess when you know you don't want to live without someone, when the thought of not having them in your life makes your heart ache, when you get all fluttery when they smile at you or say something sweet, when you spend all your time daydreaming about them, when you don't want to go more than a day or two without seeing them, when they make you mad (this is a big sign, if you care enough for them to make you mad, well...), when you forget their faults (even though they are obvious, we all have faults), when you are willing to overlook their faults (even though they bug the **** out of you), when you look at your life in 20 years and imagine them there too, when you start considering them in all that you do, when you hear something funny or sad or important and they are the first one you want to tell...I could go on and on.