You know that a guy thinks about you when he sees or does something and the first thing that comes to his mind is you.
I, as a relationship coach, believe that a guy is always genuinely happy to see you when he thinks about you. He lightens up whenever you are around him.
You two went out, and he offered to buy you a drink. He then bought you your favorite drink even though you only casually mentioned it once.
When you like a nice guy, you just can't get him off your mind; you simply would want to know more about him. It's the same vice versa.
Like an addict, your mind is always focused on the person, and you desire to get to know them and learn what they are about.