While definitive identification usually requires study of leaves, bark and reproductive bodies, different species or genera of trees have distinctive shapes. With practice, you may be able to identify trees from a distance with only their silhouettes to guide you.
Where cottonwoods and elms might have long, mostly branchless middle and lower trunks with high, upward-spreading canopies, oaks and maples often show large branches beginning quite low on the tree and forming a roughly circular crown. As D.A. Sibley notes in his "Sibley Guide to Trees," an aspen typically shows sharper, stiffer branches and twigs than the superficially similar birches.
Fir trees tend to be spire-shaped with a sharp-pointed crown; spruces have broader, conical forms. Pines and Douglas firs often have airy, well-spread boughs. The sugar pine, biggest of the pines, may have almost outlandishly long branches even at its top. Junipers, red cedars and white cedars, sequoias and redwoods are usually conical with thick-based trunks.
You can also learn something of a tree's ecological context by examining its silhouette. A bur oak seeded in a savannah or prairie habitat 200 years ago still shows evidence of that spacious nursery in a modern, grown-in wood: a broad, widespread crown. An upland tree might sport fairly short twigs, while the same species in rich bottomland may have longer ones.