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