All information pertaining to researching U.S. trademarks is contained on the USPTO's Searching Trademarks page. The Trademark Electronic Search System (TESS) is used to search for registered trademarks or pending trademark applications.
If you have satisfactorily conducted a search for a word or logo trademark and you have found nothing similar in the TESS database, your search is not yet over. You have simply determined that there is no registered U.S. trademark matching yours. You also need to search other resources such as the internet to determine if another similar trademark is being used in commerce. As mentioned on the introductory page there exist common law trademarks, while not registered with the USPTO, which still carry some legal precedent for the owner who can prove through documentation that they have been using the trademark longer.
Other resources to search include-