Your ophthalmologist will examine your pupils in both a lighted room and a dark room. This allows them to see how your pupils respond to light. This can help them figure out which pupil is abnormal.
Your ophthalmologist will also check your eyes with a slit-lamp microscope. This instrument lets your eye doctor look at your eye in small, detailed sections. That makes it easier to spot problems.
If you have other symptoms along with different pupil size, your ophthalmologist will do other tests to learn more about your condition.
Image courtesy of Steven A. Newman, MD.