There are several ways to easily identify plants--and most involve checking with a plant authority: in gardening books, at botanical gardens, nurseries and garden centers, by asking friends and neighbors, or by checking with horticulture teachers and certified master gardeners. Pictures of the plant or pieces of it--stems, leaves, flowers, roots--are necessary for proper identification.
Check out any of the multiple library books available on gardening, or borrow some from gardening friends. Lots are also available at book stores. Many gardening books contain encyclopedic information about plant types, and include photos or drawings. Some of the best books for identifying plants contain alphabetical sections of plant names, both common and scientific. These sources usually also give information about the plant's habitat, growth, flowers, height, leaves and growing season and the growing zones in which it does best. You may also try local gardening sections in publications, such as Common Ground, a weekly column Powell Gardens contributes to The Kansas City Star.
Clip off leaves, flowers, stems and even roots of the plant you're trying to identify and take them to a nursery or garden center that sells plants of all kinds. The large, greenhouse-type garden centers are the best, since employees there usually have some knowledge of the plants they care for--even if they have been grown and shipped in from elsewhere. Nurseries usually handle plants from sprout to sale-size, and have more specific information. At either place, show your clippings to people there, and they'll likely recognize at least part of the plant that you've brought in. If it's a flowering plant, usually the flower or bud is enough to identify the plant. Sometimes a leaf is enough. Bulbs, bare roots, rhizomes and other root structures also can easily help identify a plant. And a photo of the full plant helps even more.
Another place loaded with plant experts is your local Cooperative Extension Service office, which answers plant questions from the public as part of its educational mission in each state and county. Show them clippings from the plant you want to identify. Sponsored by the U.S.D.A, this agency has certified master gardeners, which it has trained to have a wide range of knowledge about plants and gardening issues. Their advice is free. Each county has a Cooperative Extension Service office, which also usually has a degreed horticulturalist on staff, and is connected with your state's agriculture department and university system.
There are more than 100 public botanical gardens in at least 46 U.S. states, and each contains a plethora of plant types, both indigenous and exotic to that locality. Horticulture experts run these facilities and often provide plant classes, special showings and seasonal events, plus an array of plant identification resources, such as brochures and gardening books they sell. Tap the knowledge of these folks by showing them the clippings and/or photos of the plant you want to identify. Chances are they can take one look and know your plant's name, and much more.
Ask a friendly local biology teacher, botany or horticulture professor to help identify your mystery plant. Even a basic science teacher usually can help with initial information, or direct you to resources with specific information.