I like people's reply so far. So many different views.
I can tell you I fell in love twice.
First time was awesome. I can tell that when I declared to her, I totally felt that "butterflys in the stomach" thing. It really feels like there are insects flying inside you. Luckly for me, she felt the same way. Just being next to her made me unrationally smile, my head sometimes felt fuzzy and light but a cool fuzzy and light feeling if that makes any sense. I could only care to make her happy and when that smile showed upon her face, it was like a star borned inside my chest every time.
When it came to an end, however, my chest wanted to rip itself apart. There was a void that nothing could fill and there were only ashes to what once was my heart.
Second time was... I don't know, I knew it wouldn't work out, that the person would never feel for me what I felt for him but you can't choose who you fall for. It was like a phoenix, reborning from the ashes, warming up my heart once again. The unrational sensation to smile whenever he was around, the heat up feelings when he laughed... it was awesome. Painfully awesome.
So, basically, love is straight down weird. You will simply know when it happens.