What is that tree, you ask yourself. It looks like it might be a walnut, but you are not entirely sure, and there are no nuts on the ground to confirm your suspicions. Is there some way you can identify walnut–with or without nuts--in all seasons? Fortunately, like most trees, walnut (Juglans species) offers clues to its identity that are easily recognized once you know what to look for.
Look at the land around you. Walnuts grow in locations requiring adequate moisture, so it is a good bet you will find them in low lands near rivers and streams. It does not like to be wet however. According to University of Minnesota Extension, "It grows best in soil that is deep, fertile, and moist but well-drained…at least 30 inches deep. …often found along streams where periodic flooding occurs in the dormant season. …not…where soils are poorly drained or wet during the growing season or where flooding or ice damage is frequent." (See Reference 1)
Examine a leaf—even in fall or winter there should be leaves beneath the tree. Walnut leaves are compound pinnately divided, (that is they contain several smaller leaves on each stem) and about 2 feet long. Usually they have from 15 to 23 leaflets. The leaflets are not parallel (directly across from one another), but alternating (like stair steps) from one side to the other.
Smell a crushed leaf. The smell of "walnut" is unmistakable.
Check for nuts on the tree or on the ground. They are hard to miss with their large green hulls—only slightly smaller than tennis balls. As they age, these husks turn black; shrink and fall away, leaving the smaller, rough black nut most people recognize.
Identify walnut in winter by cutting into a twig and examining the pith. Ostermiller says, "This pith is chambered--somewhat like a honeycomb. Only black walnut and butternut--a close relative--have pith like this. Walnut pith is brown and butternut is buff colored." (See Reference 2)
Confirm identity by breaking off a piece of bark. It should be thick and have a nice chocolate brown color underneath. On the tree, walnut bark has deep furrows and rounded ridges.