How do you know when you're in love with someone?

I've only ever truly been in love with one person. Everyone will notice they are in love in different ways. I get a very intense feeling in the pit of my stomach when he is around and even when I just think about him. When he kisses me i forget that everyone else in the world exists and when he holds my hand I feel at peace. When it hurts to be away from that person , when sometimes you can go through painful moments in the relationship you somehow come back together in the end. Eventually they become all you think about.