This is something that I've learned to observe in order to tell the difference between a toad and a potential prince. When I began to develop feelings for my best guy friend, these are the things I noticed in his behavior and body language:
He would mimic my actions. No seriously, he would do the same thing I'd be doing like let's say leaning against my chair while drinking my soda, few seconds later he'd subtly do the same. I'd walk faster, and he would pick up his pace. He'd do things of that sort that would show me that he was totally paying attention and considerate of my every single movement and thoughts.
He would apologize even when he didn't do anything wrong. Whenever he'd see me make my upset facials, he'd automatically apologize as if his life depended on it. Okay maybe I exaggerated a little, but he would apologize and ask me if he was forgiven. Let's say he accidentally stepped on my toe; the guy would automatically turn to me, hold my arm, give me this sorry look, and apologize. This definitely showed me he was really caring about being on good terms with me at all times.
We would always end up alone. So let's say we'd go out with a group of friends. At the end of the day, somehow, he and I would always end up alone drinking at some place, or at a park.
Conversation was active, and silences could be comfortable as well. We could spend hours talking about anything, and I mean anything. Talk about grass, the weather, dogs, my fear of roaches, cake, our dreams, the future, memories, but we could also feel so comfortable sharing a moment of silence.
He'd playfully push me or put his hand on my shoulder. He'd pretty much do all of the silly things just to get close to me or touch me. This signaled to me just how into me he was by not being able to take his hands off me.