The key ID characteristics I use to differentiate these two separate species--shagbark hickory (Carya ovata) and shellbark hickory (Carya lacinosa)--are buds, twigs, fruit, number of leaflets, and habitat.
Shagbark Hickory typically has five leaflets; whereas shellbark hickory usually has seven leaflets. Shagbark hickory is typically an upland species, whereas shellbark hickory is typically a bottomland species though their respective habitats tend to overlap with modest frequency. With regard to the fruit of these two species, shellbark hickory tends to have much larger husks and nuts. Shellbark hickory also has orangish-brown twigs with larger buds as compared to shagbark hickory.