You can do a search of registered US Trademarks on the US Patent and Trademark Office web site: http://tess2.uspto.gov/bin/gate.exe?f=tess&state=m41fuv.1.1 Note, however, that the US also recognizes "common law" trademarks that do not have to be registered. http://www.uspto.gov/web/offices/tac/tmfaq.htm#Search005 Of course if you want to do business outside of the US, you'll have to search the trademark databases for other countries as well.
There are trademark search companies who will search not only the public record but also proprietary databases looking for common law trademarks. Fees vary.
TM = trademark R = trademark registered with the USPTO http://www.uspto.gov/web/offices/tac/tmfaq.htm#Basic007