Red maple is different from other maples in having red flowers that emerge a week or more before the 3-lobed leaves. The red maple differs from the Norway maple and sugar maple which both have yellowish-green flowers that open along with 5-lobed leaves. Another species, silver maple, has greenish-yellow flowers that emerge well before the leaves and large 5-lobed leaves with whitish undersides. Red maples often grow in swamps.
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.)