If your mind always think of her and feel good mood all the times, then you are in love.

I usually figure it out at the point where we've done it three times in a day.

Your heart will beat faster when you think of your beloved or meet him/her.You'll get butterflies in your stomach and may lose your appetite.

If you ask me this question about 3 months ago, I would agree with the above reply. In my opinion, you can't tell if you are in love. Love is blind. Sometimes people confuse love with loneliness, lust, infatuation, or even physical attraction. Being happy is one thing and being in love is another. Both can co-exist of course.

If you can answer that question definitely, then you've done better than a few thousand philosophers and poets throught the ages. One question you should ask yourself: "Is this person's happiness essential to my own?" If you can answer definitely yes, than that's a very good start. It's not the only condition, but a big one.