You can tell a lot from someone's eyes. They're the windows to our inner self. For example, you can feel someone is confident and determined if they look directly to your face (this was the case in this book).

You can also tell if someone's smile is genuine from the look of their eyes. If you're sad, the eyes will show. If you meet someone you like, your eyes (or pupils if you will) will widen up. And other countless scenarios.