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