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.