There ar a couple ways, none are 100% though. If he talks about you, if he says your name more than usual, if he changes his posture when he sees you, if he acts generally different in front of you than with other girls. Peoples pupils dialate (get bigger) when they see something they want (or like). Test it out. Also they can't stop that even if they try so, yeah no lying there.
Youtube: how to tell if a guy likes you.
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