You will know you're in rapport when you can make a subtle change in any of these and the other persons unconsciously responds. Give it a try with a friend who is willing to give it a go. Try having a conversation about a topic on which you disagree but you're deliberately being in rapport. It is really hard to disagree! Now try it with a subject that you both agree about and be out of rapport. You'll see that it is really hard to agree when you are out of rapport.
Recently I've noticed that I'm a head nodder. My head bobs up and down all throughout conversations and meetings. I don't think I noticed it while living in the States but here I feel like a bobble head doll. This past weekend I tried not doing it and I found it really hard to stay still.
Over the past two days I've had face to face conversations with American women who are my age and definitely my energy level. You know what? They bob too. It must be an American thing, a way of building rapport. There we went, bobbing away, stepping closer to each other, matching our breathing. I hope they didn't know I was NLPing them.