His voice changes when he talks to you in a group. He may say "Hey." to everyone with you, but the "Hey." to you is a little different.
His voice gets softer when the two of you talk.
Sometimes he stares straight into your eyes.
His eyes get 'soft' when he looks at you. They change slightly or get relaxed around the edges. It's subtle, but you can tell if you watch...
When he's near you or talking with you, his eyes travel in a little circle around your face and land back on your eyes. (Like he's looking at your whole face - then back to your eyes.)
He watches your lips. (Dead give away.)
His friends start asking you questions - if you haven't met him yet - they may ask what your name is. Or may ask you what you think about 'him.'
If his friends are paying attention to you after he's been staring at you - it can mean that he's been talking about you.
He tells you that you smell nice... (No hidden meaning here - it's a good thing.)
He might act weird around you or seem very nervous. His behavior may just plain change when you're around.
If he's normally quiet, he may get louder, or vice versa. It's the difference in behavior that's the clue.