As a first line of defense, many animals avoid becoming the victim of predation through mimicry (or mimetism)--a resemblance, in physical appearance or behavior, to a species that is poisonous, venomous or otherwise harmful--thereby deceiving the predator and warding off a potential threat. One species that employs these imitative traits is the bullsnake (Pituophis catenifer sayi), which is frequently mistaken for the rattlesnake. There are, however, a number of characteristics that will help you avoid such misidentification.
Know that bullsnakes have no rattler. When threatened, bullsnakes will often forcefully vibrate their tails, which serves as a warning to potential predators. In dry leaves or grass, this will produce a sound that is quite similar to one emitted by a rattlesnake. Another related indicator is that bullsnakes will keep their tails low to the ground when producing its rattling sound; many subspecies of rattlesnake will elevate their tail when rattling.
Focus your attention on the body of the snake. Although the two often have similar patterns, bullsnakes are generally cream or pale yellow in color with brown or black markings; rattlesnakes, on the other hand, are typically much darker, depending upon the subspecies.
Know that the body type of a bullsnake is much more streamlined than that of a rattlesnake. A bullsnake will be noticeably thinner and its body will become consistently narrower down to its tail, which comes to a defined point. A rattlesnake will appear thicker, particularly in its mid-section, with a more rounded tail due to its rattle.
Observe the shape of the head. The head of a bullsnake is nearly identical in size to the upper portion of its body. The head of a rattlesnake, however, is often more triangular in shape and is perceptibly wider than its upper body.
Observe that a bullsnakes' pupils are circular, while those of rattlesnakes are vertically oriented. (The shape of the pupils is a method that many use to determine a venomous from non-venomous snake, although there are exceptions.)
Keep in mind that bullsnakes lay eggs, while rattlesnakes give birth to live offspring.
Note that although the two species may share the same den, bullsnakes tend to leave the den to warm themselves or feed earlier in the day than rattlesnakes.