Tests vary in length, type of questions, and specificity of results. They also vary in cost. A number of websites offer free placement tests, but often charge money if the test-taker wants to receive an in-depth analysis of his test results. Career centers offer placement tests, and a number of job search books include tests. Be sure to select a placement test from a creditable source in order to receive the most reliable results.
Be aware that placement tests are only one way in which to discover the career that is right for you. Ultimately, experience (either through job shadowing, an internship, or an informational interview) will tell you whether or not a specific career is a good fit.
Career Tests: Articles and Advice