ok here we go
well for starters, you have a feeling. not a feeling in your pants, but a feeling of love and appreciation. you will be able to know you are in love with someone when you are able to look past their physical attractions and deeper into the true person they are. you won't care hardly as much about what they look like or how good they are in bed.
you will want to cherish every moment you spend with them, not because you enjoy flirting, but you enjoy them as a very very close friend.
love is usually a two sided thing. each person loves their partner. when this happens, the love grows stronger. but if one person loves another but the other person doesn't feel the same way, then its more of a physical attraction usually, but sometimes it is actually love that was not meant to be.
you might not know when you are in love. REAL love until that person is no longer accessible to you, for example, they get a boy/girl friend, and only look at you as a friend, they leave and/or move somewhere else.
you often won't realize when you are truly in love.
that doesn't mean that if you think you are in love, then you are not. you might be right, but you might also be thinking irrationally and blowing things out of proportion.
all i really mean to say, is you will have this feeling of peace when you are with them. and you will never want it to end.
