Don't know how I missed this post; I usually jump on posts like this!
There are 4 basic groups of venomous snakes in the US. The rattlesnakes, the copperheads (yep, more than one species), the cottonmouth or water moccasin, and the coral snakes (2). The first 3 are all pit vipers, so called because of the heat sensing labial pits located between and below the eyes and nostrils. Along with the "cat's eye" pupil mentioned, this is the easiest way to tell (at a safe distance) a venomous snake. The pits are clearly visible. I'll try to post some close up head shots of both a venomous and a non-venomous snake for comparison in a few days (no time this weekend!). On the average, the pit vipers can strike about 1/3 rd their body length. While the Eastern Diamondback can reach lengths of 9 feet, they are EXTREMELY rare. Figure that a large rattler is 4-6 feet in length. That means the average strike range is 2 feet. In all but the thickest brush, avoiding being bitten simply involves being careful where you step, or where you place your hands. Don't rely on rattlers rattling every time. Sometimes they don't, and sometimes they've lost their rattles to rocks, etc. Another thing is that a defensive bite often means a dry bite. No venom delivered. Venom is a food capture tool and is precious to the snake. It won't use it unless it has to. None of these snakes are aggressive; they just want to be left alone. Best to leave 'em be. I'd rather have snakes around than mice and rats anytime.
On to the coral snakes. These guys are a whole different ball game. They don't have the elliptical pupils or labial pits, but they're very easy to identify and the rhyme posted below does that nicely. Corals are secretive and shy, and they're not easy to find. I spent an entire week in Florida looking for them and never found a one. They're also small and have difficulty biting anything other than fingers and toes. Usually, it's kids that get bitten when they're attracted by the pretty colors.
Venoms are peculiar and complex things. They're a cocktail of enzymes and proteins that attack various parts of the body. In general, and except for the Mohave Green rattler, N. American pit viper venoms attack skin, muscle, and blood, while the coral and Mohave Green attack the central nervous system. These don't have the gross, nasty visible effects of the pit vipers, but they can shut down breathing and stop the heart.
All in all, snakes (even the venomous ones) are beneficial critters and should be left alone unless they're a direct threat to people or pets. I realize you can't have a rattler in your back yard playing with your kids or dogs, but when you encounter one in the boonies, you're in no danger unless you get really stupid. Leave it be.
By the way, Indigo's have become really rare and to my knowledge, are one of only a few species of snake on the federal endangered species list. Also, catch or kill an Indigo on a farmer's property in Fla., and you risk a 12 ga. round of rock salt!
I caught a 7 footer in a trash pile in Fla. in the 60's and the property owner caught me and threatened to have me arrested for trespass. Needless to say, I let the snake go.