1. Observe his posture.
Male humans, like many mammals, change their physical stature when they are trying to impress a female of the same species.
He won't have his arms or legs crossed. Crossed arms and legs are body language for "no, stay away!"
He may lean in towards you while talking to you, or turn so his body is facing yours.
He will likely stand straight and tall with square shoulders to seem taller and more impressive.
He might even seem to be thrusting his pelvis out slightly. Alternately, some men subconsciously (or consciously) adjust their genitals when speaking with someone they are interested in.
2. Look for facial cues like blushing and smiling. A boy who is interested in you will likely spend a lot of time smiling at you. He'll also laugh at your stories, even if they are not very funny.
If a boy is shy, he will likely blush when you talk to him. Look for any redness on his cheeks or signs of sweaty palms (he may rub his palms on his clothing or fidget with his hands).
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