Ashes are notoriously difficult to tell apart for the novice tree student. But they're nowhere near as hard as Willows!
Ashes have compound opposite leaves. Their branches are very obviously opposite when viewed against the sky.
This photo shows the distinctive form of Ashes (in general) in the winter. Notice how the twigs are stout and obviously opposite.
Maples are the group usually confused with Ashes in the winter. Their twigs are much less stout, almost scraggly. In fact, sometimes one has to search to find an obviously opposite pair.
This is a White Ash (Fraxinus americana).
To see all Ashes in order by Common Name →