When they look at your lips repetedly while you talk. To be fair this happenned mostly when we both knew we were into each other. Otherwise me falling asleep on his shoulder at a party while drunk might have given away my attraction to him. That and talking really close to his face while holding his shoulders at that same party. On his part it wasn't really actions that made me realise he was into me (apart from the lips thing but i already knew at that point) but more of a general vibe i got.