Actually to be honest, all the respuestas before me are part of love, but it is also part of attraction to the person. I mean one does get mariposas and feels nervous around a crush, but that does not mean tu would feel the same way when tu are in amor with someone and they are in amor with tu back.
Truth be told, the feeling of being in amor is different for everyone. The same common feelings like wanting to be with the person always and all that mariposas are still there, but when tu are in love, it all intensifies, beyond what tu imagined could ever happen before.
But above all else, being in amor means putting someone before yourself and also telling yourself that I am worthy of such amor from this person and I as a person would be able to give that amor back. It's all about feeling that ability to give and receive as well as to care, sacrifice and respect.
tu know you're in amor when tu think about that person all of the time, when you're corazón starts beating faster every-time tu see him/her. tu want them so badly it makes tu want to scream. That's how tu know.
You're in love- and I mean it- when tu can imagine yourself spending your days at the side of your loved one. When tu don't mind making sacrifices, when tu know, tu know, cause your corazón tells tu and your mind agrees.