Pit vipers are a bit more difficult. Of course the presence of rattles tells you that you are dealing with a venomous snake, but absence thereof gives no assurance to the contrary. Copperheads and cottonmouths have no rattles, and even rattlers sometimes loose their rattles. The presence of fangs indicates a venomous snake, but these may be folded back in the mouth and difficult to detect. The fangs may even be broken off. The easiest indicator (but one which requires practice, maybe in a zoo) is the characteristic heavy body and triangular head of the pit viper. Although some nonvenomous snakes also have these characteristics it is always best to treat a snake with caution.
In a dead snake you could look for the pit after which the pit viper is named. This will be between the eye and nostril, one on either side of the head. Another sure indicator is the scales behind the anal plate. Pit vipers have a row of single scales reaching across the underside of their bodies behind the anus while most other snakes have a double row of scales, joining in about the middle. Some references suggest checking the pupils of the snake's eyes for identification. Pit vipers will have vertical slit pupils. If the snake has round pupils it is not a pit viper. These fine characteristics are probably only useful in identifying a dead snake. One wouldn't want to pick up a live one to look at its underscales or its pupils.
One good indicator of the type of snake is the location. Snakes don't wander far from home. They tend to have a limited range and will not survive outside the conditions they prefer. Except in the case of an exotic pet you will not find them in areas far removed from their normal range. You simply won't find a Mojave rattler in Ohio or an eastern diamondback in Colorado. Several of the books listed above describe the ranges for various species, usually with maps.
Body markings are rarely sufficient for identification by the inexperienced. It takes a lot of practice to learn to distinguish between various species, some of which are quite similar externally. Furthermore, individuals of the same species can have varying shades of color, making such identification even more difficult. See the references listed above for pictures and other help in identification. If you are concerned about venomous snakes, get a book on herpetology and study it. Visit zoos and talk with specialists. You will not become an expert by reading the Internet.
Again, the best rule is, leave snakes alone! Getting close enough to identify pits or scales is dangerous. You might then be able to also examine the fang marks on your body!
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