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The most commonly used diagnostic tool for TB is a simple skin test. Although there are two methods, doctors consider the Mantoux test the more accurate.

For the Mantoux test, a small amount of a substance called PPD tuberculin is injected within the skin of your inside forearm. You should feel only a slight needle prick. Within 48 to 72 hours, a health care professional will check your arm for a local reaction to the injected material. Depending on your response, the test is diagnosed as positive or negative. A positive response — usually shown by a hard, raised bump at the injection site — means you're likely to have TB infection.

Taken from www.mayoclinic.com