You can identify backyard snakes, and other snakes, by visual, behavioral and environmental indicators. A good regional field guide, such as the National Audubon Society Field Guide to Reptiles and Amphibians of North America, lists snakes in the area and emphasizes distinguishing marks.
Snakes exhibit a wide array of physical features. Body type and size are good indicators. Check to see whether your specimen is slim or stout, long or short. Note colorings and markings as well. Patterns found in the snake world include rings, blotches, diamonds, stripes and speckles. These markings are usually a distinct color from the rest of the snake. Some snakes also have a different color on their underbellies than on top.
The shape of a snake's head is another aid, though less reliable. Check to see if it is oval shaped or triangular, while also noting that some species have more or less pliable heads.
Behavioral indicators that aid in identification include tail vibrations, tail rattling, and, if observable, the type of prey and the manner of killing the snake utilizes.
As snakes inhabit different environments, noting which snakes live in backyard-type habitats (whether grass covered, wooded or by a body of water) can go a long way toward narrowing the list of possibilities. Range is another factor. An updated field guide to snakes contains range maps or descriptions alongside each entry.