He/she is the only guy/girl you see and you don't want anybody else. Also if you can be yourself around that boy/girl then you know it's real love. Basically love never can be explained, we know that love is based on everybody's perspective. So there isn't a specific to know when your in love. Somehow you just know it's right :)
Hope this helps!
I'll let you know when i feel i'm in love
If s/he's almost always on your mind, if you relate music to your situation regularly, you get butterflies a lot when you think about meeting her/him.