There are only four venemous snakes native to the United States: the rattlesnake, copperhead, water mocassin and the coral snake. In these species there are numerous subspecies and their appearance can vary based on that. When I was growing up my best friend's dad was a naturalist and herpetologist and I spent a large amount of time assisting him in collecting snakes and other animals for wildlife shows he gave to various groups. I also assisted in teaching about them and we always taught people it was easier to know how to recognize the poisionous ones that way if the one you see is not one of them, it should be non-poisionous. This is dependant on proper identification of course. Like Greg Walters said in his reply, most snakes look different when young. An example is the water mocassin. They are normally a grey to black mottled color when adults, dependant on sub-species, yet when young they resemble a small copperhead. When my wife and I started dating I knew she was the one for me when she agreed to go water mocassin hunting with me and my friends and subsequently went on several other trips. If you have never done this it entails wading a creek late at night. We have several creeks in southern Oklahoma that are very productive. The most we got in one night was 23 and one of them was 51 inches long. All of them got released at the end of the night except for the biggest one and he has been used in many shows and exhibits. With four young kids now, I rarely get to go. Sorry, forgot to add that neither blue nor black racers are poisionous. At the end of all the classes we always told people that if you cannot positively identify the snake assume it is poisionous and keep your distance.
). I do have some veeery paranoid ground squirrels though. Too big for her to eat, too small to feel really safe!! Neighbors have no blacksnakes and bunches of Copperheads. 
