From an article I found at Southeasteroutdoors.com:
Identifying Snakes
There are two families of venomous snakes native to the United States. The pit vipers, of the family Crotalidae, which include rattlesnakes, copperheads and cottonmouths, make up the vast majority of venomous snakes in the country. Pit vipers get their common name from a small heat-sensing pit between the eye and nostril that allows the snake to sense prey at night.
The other family of venomous snakes is Elapidae, which includes two species of coral snakes found chiefly in the southern states. They are related to the much more dangerous Asian cobras and kraits. Coral snakes have small mouths and short teeth, which give them less efficient venom delivery than pit vipers. People bitten by coral snakes lack the fang marks of pit vipers, sometimes making the bite hard to detect.
Here are a few guidelines to identify most venomous snakes.
Pit Vipers
* Head Shape: large, triangular shaped head and much narrower neck
* Pupil Shape: vertical or elliptical pupil, like a cat's eye
* Presence of Rattles: rattlesnakes only
Coral Snakes
* Head Shape: slender
* Pupil Shape: round
Coral snakes share many characteristics of non-venomous snakes. There are many variants on sayings that will help one remember the difference between a coral snake and a harmless king snake or milk snake.
"Red on yellow, kill a fellow; red on black, venom lack."
"Red next to black is safe for Jack; red next to yellow will kill any fellow."
"Red on yellow, dangerous fellow; red on black, friend to Jack"
"Red on yellow, kill a fellow; red on black, won't hurt Jack"
All tell the same warning about the color pattern for North American coral snakes. However, identifying venomous snakes isn't easy; there are numerous exceptions to the rules above, especially when dealing with coral snakes. Characteristics such as head shape are subjective to the observer; something that appears triangular to one person may seem slender to another. And some nonvenomous water snakes have characteristics that make it easy to confuse them with venomous snakes. Also, identifying some of these characteristics, such as noting the pit sensors, requires close examination of the snake, something we recommend you avoid.