To find out if a trademark has been issued in the United States, search the Trademark Electronic Search System (TESS)

What are Trademarks and Service Marks? 

A trademark ( ™ ) includes any word, name, symbol, or device, or any combination used, or intended to be used, in commerce to identify and distinguish the goods of one manufacturer or seller from goods manufactured or sold by others, and to indicate the source of the goods.  In short, a trademark is a brand name.

A service mark is any word, name, symbol, device, or any combination, used, or intended to be used, in commerce, to identify and distinguish the services of one provider from the services provided by others, and to indicate the source of the services.

A registered symbol ® can only be used on goods or services that are the subject of the federal trademark registration.

For more information on trademarks see the Trademark tab on  USPTO's frequently asked questions (FAQ's).

About.com also has a nice overview on trademarks for inventors.