Shells

A close look at the shells of various acorns can help pinpoint the species. Black oak acorns feature hair on the inside of the shell. White oaks produce shells with smooth surfaces inside.

Cups

Some trees, such as the bur oak, grow thread-like fringe on the cup. The pink oak features a shallow cup that resembles a saucer. Top-shaped cups may indicate a bear oak. Knobby-looking scales found on the lower part of the cup suggests the acorns comes from white oaks. Scarlet oaks feature fruits that are half-covered by a deep cap.

Crop

White oak trees produce acorns every fall, so you won't find immature acorns on white oaks during the winter. Black oaks produce fruit every other year in the fall, so you can see immature acorns on the tree, even in winter.