Hybridization of maple species
Usually, a positive identification of a maple tree can be made with a tree field guide and a close look at the bark and leaves of the tree.
However, if you're really perplexed about the identification, you might be looking at a hybrid maple, a cross between two different maple species. This can happen in areas where two maples species bloom simultaneously. It occurs commonly enough that an entire chapter of the book, Maples of the World, is devoted to hybrid maples.
Black and sugar maple hybrids
In the eastern United States, hybrids of black maple (Acer nigrum) and sugar maple (Acer saccharum) are common, according to the U.S. Forest Service Sylvics Manual and other sources.
The black and sugar maples have crossed and backcrossed so many times that the two species are nearly indistinguishable in some areas. In other areas, the black maple still retains its own unique characteristics.
There appear to be two broad populations of black maple with respect to its hybridizing with sugar maple. One is in the western part of its range, where it maintains its identity and shows little tendency to cross with sugar maple. The second population is in the eastern section, where it hybridizes readily with sugar maple. (Source: U.S. Forestry Sylvics Manual)
Red and silver maple hybrids
George Ware of the Morton Arboretum writes "Natural hybridization between red and silver maples is fairly common in the swamp and streamside forests of eastern Wisconsin and eastwood to New England " (source: pdf). Hybridization is also reported in southern Canada. In these regions, red and silver maples bloom at the same time, and the trees grow in similar habitats.
Because the blooming periods of silver and red maple overlap, there is a possibility of natural hybridization between them. Under controlled artificial conditions, the two species hybridize easily, producing prolific seed sets. The hybrids are intermediate between their parents in leaf characters. Their growth was much faster than that of red maple seedlings but did not equal that of silver maple. Source: U.S. Forest Service Sylvics Manual
In fact, red/silver hybrids have a name -- Acer x freemanii or Freeman maple -- and a number of named cultivars, including Jeffersred (patented as "Autumn Blaze"), Armstrong, Celzam (patented as "Celebration:), and others. The name Acer x freemanii is pronounced "Acer hybrid freemanii".
Related: A comparison of the leaves of red, silver, and Freeman maples