Good answer Tartanwizard! To add to it, I'd say that you know when you trust that person with your life, would never doubt them, and wonder how the hell you ever managed without them being around you, before you met them.
When you are first in love you feel it in your stomach, queazy and fluttery. You don't want to trouble it with food because its just not important. All you want is that person and to see them, be with them, hear them.The anticipation! It's a lovely feeling, but one which physiologically is based on sexual attraction. Real love then calms down into the feeling of belonging together, even when you are apart, and having a sense of mutual trust and respect. And still having fun!
Thinking of him/her makes you smile.. even on a crowded bus. If youre not with them, you imagine them lying next to you at night. You feel dreamy and frivolous most of the day and dont care much about anyone else around you. You find it hard to concentrate on work because your mind keeps wandering off to the thought of them, the things you have shared and the things you will share. Most important of all, it makes you feel purely happy.
xyxxy, I would think that my and TW's description of the symptoms are exclusive to the feelings of love. However huge a crush I have on Ben Affleck, I wouldn't think of him all day, let alone imagine him lying next to me at night. No, really I wouldn't.