Only you can really answer that. But I would say when you do something and it reminds you of that person, or when they leave you, you feel like a part of yourself is missing. They truly are your world, and you're theirs.
You feel it in your heart. Also, you think about that person like every minute of the day. You smile when someone mentions their name. You can't wait to see that person again. You want to make that person happy no matter the cost. You would do anything for them. You would just be happy all the time. You will sing at the love songs on the radio. You will never see the bad in a day because you saw that person smile. You feel your heart beat out of time when they come near you. You can just feel it. So, good luck with whoever you are in love with.
you get butterflies when they texts you or touches you, or even looks at you. you'd wanna spend every last second you could with them. you'd sit in class not paying attention thinking of him/her. staying up every night just to talk to them or think about them. you'd always talk about him/her. you'd do anything for them. risk your life even. dream about them. know every last detail about them and still except their flaws and mistakes. love them for who they are and not what you want them to be.