Watch to see if he stares at her for long periods of time. If he gazes at her longingly after the conversation has ended, then he is probably attracted to her. Try to catch him staring at her, and watch to see if she stares back. If he keeps staring, he is most definitely attracted to her.
Watch his body language. If he moves closer to her, places his hand on her shoulder or leans in to talk to her, then he is probably attracted. According to the website "WhatDoMenReallyThink.com," a man will mirror what a woman does if he is attracted to her. If she moves forward, he moves forward; if she takes a drink, he takes a drink.
Look to see if the man's pupils grow when he talks with a woman. If they grow large and stay that way, he is most likely attracted to her. According to "DatingAgain101.com," evidence shows that our pupils grow large when we see something we like and dilate when we see something we dislike.
Does the guy try to make the woman laugh, or do silly or outlandish stunts to get her attention? If so, he is probably attracted to her. He wants to prove to her that he is strong, funny and intelligent. A woman will respond pleasantly to this behavior if she is attracted to the man.
Watch to see if the man seems extremely helpful to the woman. If she needs something, does he help her get it? Does he always get the door for her, does he always seem to be there when she needs a hero? If so, he is attracted to her.