The aboral side of the shell (test) has the off-centered petalidium, as seen clearly here in a dead specimen which has lost its spines. Note also that, unlike urchins which have 5, sand dollars have only 4 gonopores, visible around the star-shaped pattern in the center.
Eccentric Sand Dollar (Dendraster excentricus), Ganges Harbour, Salt Spring Island, British ColumbiaSource: http://en.wikipedia.org/wiki/File:Eccentric_sand_dollar.jpg
The oral (under) side of the test is flat. It has the mouth in the middle, with tracks leading to it. The anus is near the edge of the shell (at the bottom in this photo).