You can know that you are in love if you feel attracted to the person, caring for them and always wanting to be by his or her side. When you are not around this person, you will be longing for him or her and most of your thoughts will be occupied by the person. See http://www.ehow.com/how_4424394_know-youre-love.html for more tips.
You feel like someone is tickling your heart, you feel funny in your belly and you smile like a cute idiot when you are all alone or even with people around for no reason at all :p and many other signs for sure. Since you asked you already know :) Good luck huny Love is beutiful <3 .^-^.
When you think about them every spare second of the day (and busy seconds as well)...when your stomach does a flip if you see them, or if they see you, or when they walk past you...when the world seems brighter when they're around...when you find yourself gravitating towards them whenever you're near...you're in love with him, no doubt about it. Or, as the great Dean Martin said (or rather sang): When the moon hits your eye like a great pizza pie When the world seems to shine like you've had too much wine When the stars make you drool just like pasta fazool When you dance down the street with a cloud at your feet When you dance in a dream but you know you're not dreaming Bells will ring ting-a-ling-a-ling, ting-a-ling-a-ling And you'll sing "Vita bella" Hearts will play tippy-tippy-tay, tippy-tippy-tay Like a gay tarantella That's amore!