What head feature can sometimes aid in identification, although it may be faint?

Answer

A dark line or stripe that runs from the eye back toward the jaw angle.

A common, though sometimes faint, feature aiding identification is the presence of a postocular stripe running from the eye toward the angle of the jaw.

What head feature can sometimes aid in identification, although it may be faint?
animalreptileserpentviperRattlesnake