Two places to check for an answer key to a government textbook are either in the back of the book itself or in a separate volume. Check the textbook publisher's website or call the company to learn if it offers an answer key as a separate volume.
An example of a publisher that offers government text answer keys in a separate volume is Seton Educational Media, which publishes its "American Government in Christian Perspective" with a partial answer key sold separately. As of 2015, answer keys are available to both the second and third editions of the book, which users of the book should note before purchasing the answer key. A Beka Book also publishes a separate answer key for its American government textbook.
An example of a publisher that includes the answer key in the back of its government textbook is Carson-Dellosa Publishing, which it does in its "United States Government, The 100+ Series, Grades 5-8." Users can find the answer key's location in the table of contents, which is available online at ChristianBook.com.
Some textbooks have an accompanying website, which is the case with McGraw-Hill's "U.S. Government: Democracy In Action." In addition to providing chapter summaries, this website also has self-correcting quizzes that reveal the right answers after quiz completion.