Cataracts may be noticeable early in the ophthalmic exam because of a dulled red reflex. This is caused by cataracts scattering the light that enters the eye. However, sufficient light still enters the eye and cataracts themselves thus do not lead to relative afferent pupillary defects.
The type and severity of a cataract are best judged on slit lamp exam. Nuclear sclerotic cataracts, such as depicted below, are seen as a discoloration of the ocular lens.
A video explaining the appearance of nuclear sclerotic cataracts can be viewed here (at 2:00).