Taking practice tests and studying road manuals are methods of studying for road sign tests. USA-Traffic-Signs.org and DriversLicenseTest.net both offer several multiple-choice-question road-sign practice tests. The Federal Highway Administration also has an online manual with all the U.S. road signs and their meanings.
USA-Traffic-Signs.org has a five-part practice test based on information from the Federal Highway Administration. The questions are all multiple choice. The site grades test results and provides the right answers after the test.
Driving-Tests.org has a practice road signs test with 40 true/false questions. The answers are shown after each question, as well as the score and the number of mistakes permissible to still pass the test. All the information in the test is based on the DMV manual. The site also offers a Beginner Driver's Guide with articles about various aspects of driving and car ownership.
Finally, DriversLicenseTest.net has four different road-sign tests, all with multiple-choice questions. In contrast with the other sites, DriversLicenseTest.net has both multiple-choice questions that ask users to match a meaning to a given sign and questions that ask users to choose a sign to match a given meaning. Questions may also focus on the kind of sign, rather than the specific meaning of the sign. The tests are graded at the end, and the correct answers given.
For additional study material on U.S. road signs, the Manual on Uniform Traffic Control Devices from the Federal Highway Administration is available online and includes pictures and explanations of the various road signs.