Leaves have five lobes with few teeth and are pale beneath. Leaf buds are brown. Sap from a broken leaf stalk is not milky. Mature bark is deeply furrowed or shaggy. (The Norway maple has milky leaf sap, green leaf buds, and the tight bark has narrow groves) Fruit is a two-winged samara. In autumn these trees change color to scarlet, orange or yellow. Found in woods, and often planted.
There are four different ways to identify the two species of maples:
Leaves: In full leaf, red maple leaves have 3-5 lobes with many fine teeth along the edge of the leaf; sugar maples are often 5-lobed with fewer, larger teeth and the notches between the lobes are smooth and untoothed (Red = Rough, Sugar = Smooth). (Also, the leaf stalk of sugar maple will produce a clear sap after being plucked, whereas the Norway maple produces a milky sap.)
Bark: Red maple has lighter, smoother bark than sugar maple.
Buds: Red maple buds are red and knobby and sugar maple buds are brown and pointy.
Flowers: Both species flower before they leaf out, so in the early spring the easiest way to tell them apart is by the flowers. Red maples have prominent red flowers that are visible even from a distance. The flowers of sugar maple are yellow to green, and less showy. (The flowers of sugar maple hang down, whereas Norway maple flowers are produced in a spreading bouquet.)