There are three main steps to a subject search to determine if an idea has already been patented:
If the correct classification designations for your invention have been selected and there are no conflicting patents on any of the lists, a patent for that invention has not been issued in the United States.
1) Consult the Index to the U.S. Patent Classification.
The Index, used to access the Manual of Classification, is an alphabetical listing of subjects contained in the Manual. Select a word which best describes the subject of the invention, e.g. bicycle seat. The Index will list one (or more) relevant class and subclass numbers. If there is more then one class or subclass listed for the subject, choose the one that best represents the subject matter of interest. The main heading "Bicycle" in the Index has many subheadings, one of which, "Seat," refers the user to class 297, subclass 195.1+. Remember that a plus (+) sign next to the subclass indicates that there are other subclasses listed under 297/195.1 in the schedule that need to be examined. If you can not locate an entry point for your idea in the Index, try doing a key word search in one of the patent databases to locate previously issued patents that are similar to your idea. See what class these patents are in and determine if this is the proper class for your idea.
2) Turn to that class/subclass in the Manual of Classification.
The Manual is the complete listing of classes and their subclasses, arranged by class number in ascending order. If the Index provided both a class and subclass number, turn to that class and begin the search at the subclass provided. If the Index provided only a class number, start with the first mainline subclass (in capital letters) in the selected class and proceed from one mainline subclass to the next until one is located that includes the subject matter of the invention. Then, starting with the one dot subclasses, examine the schedule of subclasses beneath that mainline subclass (assuming there are some) for the appropriate subclasses for the invention. In some cases, the numbers provided by the Index will be the only ones needed, but this is very rare. In most cases, a scan of the Manual will indicate more specific class/subclass numbers for the invention. Remember to always pay attention to the scheduled order of the subclasses. If the definition of a class or subclass is required, consult the Patent Classification Definitions.
3) Search each of those classification numbers.
Decide which of the classification designations you have selected will require investigation. This is usually the designation that is identifying the most important aspect of the invention. Obtain the list of every patent issued to each of those classifications and examine every patent issued to them. If none of the patents on the list(s) are your exact idea, and you are confident that you have been searching in the correct classification area, then your idea has not been patented in the United States yet.
Patent Searching Tutorial at the University of Texas
This is a well organized tutorial on how to search for patents, created by Susan Ardis at the University of Texas at Austin. By following this tutorial you will learn how to conduct an invention search, and by the time you are finished, you should be able to conduct a basic search to determine if your invention has already been granted a patent in the United States.