Most tree identification books or keys start by observing the arrangement of the leaf, limb and bud. It is usually the primary first separation when identifying most common tree species in dormancy (in winter). You can eliminate major blocks of trees just by observing its persistent leaf and twig arrangement.
Trees with alternate leaf attachments have one unique leaf at each leaf node and typically alternate their direction along the stem. Trees with opposite leaf attachments pair leaves at each node. The whorled leaf attachment is often considered an opposite attachment in most tree keys. Whorled attachments are where three or more leaves attach at each point or node on the stem.
The opposites are maple, ash, dogwood, paulownia buckeye and boxelder (which is really a maple). The alternates are oak, hickory, yellow poplar, birch, beech, elm, cherry, sweetgum and sycamore.
Alternate and Opposite - USFS Image