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