You'll need headphones for the test. For better results you should take the test in a quiet room and use headphones that won't allow you to hear external noises. Disclaimer : Be aware these type of tests (there're several online) are not accurate, some of them can even hurt your ears, and you should always have your hearing checked by your doctor anyway.
http://www.hear-it.org/Online-Hearing-Test
http://www.hear-it.org/Online-Hearing-Test
