Eastern Coral Snake, Micrurus fulvius
Eastern Coral Snake, Micrurus fulvius (DANGEROUS: DON'T YOU HOLD ONE!)
Few poisonous snake species occur in North America, and they are all easy to identify as venomous. Several non-venomous snakes mimic the venomous ones. The venomous ones fall into two groups:
Coral Snake distribution in the United States

Of course, if you live within easy walking distance of  a more or less natural area where pit vipers are known to occur, you won't want to approach any unknown living snake to see if its pupils are convex and if it bears heat-sensitive pits. To identify venomous snakes from beyond striking distance you must study pictures of the poisonous species in your area and commit the details of their appearances to memory. There is no short-cut. Pit vipers do tend to have more or less heart-shaped heads because of the bulging poison glands behind their eyes. However, many harmless snakes also have more or less heart-shaped heads.