Members of the pit viper family, rattlesnakes, copperheads and water moccasins, have broad triangular heads. Non-venomous snakes have narrow rounded heads. Pit vipers have elliptical shaped pupils and prominent curving fangs. Also, pit vipers have a deep pit located between the nostril and the eye, thus the term "pit viper."
For more information, please read the article How to Recognize a Poisonous Snake