In the US there are very few kinds of snakes that can hurt you and most of them have rattles.
Out of those that don't have rattles, only the cottonmouth and copperhead really strike, and they have the same head shape as all the rattlers.
The other poisonous snakes have to really chew on you, they don't have large fangs, and don't inject poison.
So really, all you have to do is look for rattles and head shape.
All rattlers and the copperhead and cottonmouth have a kind of triangle shaped head.
RattlerCottonmouthCopperheadThe other snakes that are poisonous (several kinds of Coral snakes) are pretty damn easy to spot, and really, they have very small teeth. They would have to chew on you for awhile to get to blood and their poison just kinda seeps from their mouth into the wound.
ETA: My Cottonmouth pic needed replacing.