Looking at the pupils is an easy way to tell what kind of lens you have (other than just looking at the focal length). A quick peek at the entrance and exit pupil of a lens will give you the information necessary to distinguish between the various lenses.

This process works because the principal planes and the pupils are closely associated with each other. When the principal planes move, the pupils will change size and move also.

Retrofocus lenses have the principal planes shifted toward the back of the lens and toward the camera's detector. When the rear principal planes moves back, the exit pupil will enlarge. The front principal plane also tends to be shifted backward. This will cause the entrance pupil to be smaller. With retrofocus lenses the exit pupil is larger than the entrance pupil - often a rather dramatic difference.

Telephoto lenses are just the opposite since the principal planes are shifted in the opposite direction. The entrance pupil is larger than the exit pupil. Symmetric lenses will have pupils of similar size.