Texture is the most useful bark clue, because you can feel it as well as see it.
Bark texture
Ridges and furrows
Deep, long grooves are the commonest texture on mature broadleaf trees. Ask whether the ridges run straight up the trunk, as on many oaks and tulip trees; cross to make diamonds, as on white ash and black walnut; or twist, as on black locust. Whether the ridges are flat-topped and pale-streaked (northern red oak) or narrow and rounded also helps.
Plates and blocks
Pines and some oaks crack into flat plates. Ponderosa pine plates are large and orange-brown, separated by black fissures, and look like jigsaw pieces. Flowering dogwood bark breaks into small square blocks, sometimes likened to alligator skin. Black cherry bark on older trunks has thick, upturned dark flakes.
Scales and flakes
Small, thin scales that flake off are typical of spruces and of many apples. Silver maple bark peels in long curling flakes, and sycamore flakes in patches to show creamy white beneath.
Peeling sheets
Birch bark peels in thin horizontal sheets. Paper birch is white and papery, river birch is salmon-brown and curls in ragged layers, and yellow birch peels in thin golden-grey curls. The peeling is easiest to see on branches and young trunks.
Strips and fibres
Shagbark hickory has long plates that curl away at the ends. Eastern redcedar, northern white-cedar and western redcedar have stringy, fibrous bark that pulls away in narrow strips. Bald cypress is similar.
Smooth bark
Smooth bark does not mean a young tree. American beech stays smooth and pale grey for its whole life, and aspen stays smooth and pale with dark patches until it is old. Beech bark is also easily scarred by carved initials, which stay visible for years.
Feel it
Run your hand over the trunk. A rough texture that you can catch your fingers on points to ridged bark, while fine flakes that come away on your palm point to scaly bark. Take care, and do not peel living bark off a tree.