I have taken some time over the last two winters to pick up hickory nuts and do some illustrations, here they are. I know that these would be much more valuable if I were able to add scale to the images, but at this time I am not.
Bitternut hickory (Carya cordiformis) these small nuts are about 2-4 cm tall with a thin husk. Notice in the top right image the sutures are slightly winged - most of the other hickory species have smooth sutures. Pecan (Carya illinoensis) has winged sutures - I have not sketched pecan yet...
Pignut hickory (Carya glabra) these nuts are slightly larger (3-4.5 cm) than bitternut hickory and have a distinctive pear-shape. The actual nut surface is smoother and less angled than bitternut hickory and shagbark hickory. Pignut hickory and sweet pignut hickory (Carya ovalis) are difficult to tell apart, however the most often described difference is in the mature fruit. The difference: the husks of pignut do not split all the way to the base of the fruit however occasionally they will along 1 of 4 sutures, sweet pignut husks split all the way to the base along all sutures when mature (see below)
Fruit of sweet pignut hickory (Carya ovalis) similar shape and size as pignut but with sutures splitting all the way to the base of the fruit.
Shagbark hickory (Carya ovata) fruits are about 3-5 cm in length they are oval to round and do not have a distinct pear shape. The husk is very thick (3-12 mmm) and freely splits all the way to the base. The actual nut is strongly angled. The insides are tasty.
Last but not least is shellbark hickory (Carya laciniosa) often times called kingnut hickory the fruit of this beast are 5-9 cm in size. The husk of the fruit is also very thick 8-12 mm. Also very tasty on the inside.