What is the singular confirmation needed to identify any encountered snake as a rattlesnake?

Answer

The presence of the tell-tale rattle at the tail tip

The presence of the tell-tale rattle at the tail tip is stated as the singular confirmation needed to know you are dealing with a rattlesnake, regardless of the species' coloration or pattern.

What is the singular confirmation needed to identify any encountered snake as a rattlesnake?
faunareptiledesertsnakeRattlesnake