The whites of the eyes turn yellow, as do the insides
of the ears and the relatively hairless part
of the belly between the hind legs.
In these areas where the hair is either extremely thin or almost nonexistent it is much easier to see the skin, and therefore easier to tell whether or not the skin has that tell-tale yellowish color.
Maybe you could try putting a Buster Cube or other "educational" toy in with her.