What are the signs of a guy liking you but he doesn't express it?

Ok, so, people, do you know any signs of a guy liking a girl but he just doesn't express it and keeps it as a secret. This one boy in my homeroom, he had a crush on me and I couldn't believe it because we rarely talk or know each other at all. Someone told me his secret and I asked him, he confessed. You see, I want to know the signs. I hope you are able to answer my question. =)