Understand the process. Current polygraph testing device accurately measure one's blood pressure, heart and respiration rate, as well as other factors to determine if there are any changes based on questions asked. Such changes can very well indicate that one is not being truthful. In law enforcement, not everyone can give a polygraph or lie detector test. They are given by trained professionals, like police officers, and can successfully help ascertain guilt or innocence.
Know the required qualifications. Not all police officers are automatically qualified to administer polygraphs. Once you become a police officer, you'll have to specialize in lie detection and get certified in this area.
Get your lie detection certification. In addition to the above criteria, you will have to take an accredited polygraph course and complete at least 36 credit hours. Several associations like the APA or American Polygraph Association, the AAPP (the American Association of Police Polygraph) and the ILPE (International League of Polygraph Examiners), offer authorized certification programs. (See Resources)
Apply for certification and take the test. Once you complete the above requirements, you'll need to apply for certification with one of the above associations. You will then be required to have completed your coursework (within 3 years from starting program) and you will have to pay a fee of $50 or more (depending on which association you apply).
You'll then complete the certification exam that is administered by a licensed polygraph authority. After you pass, you'll become a certified police officer giving lie detector tests. Note: You will be required to renew your license every 3 years and pay a nominal fee of $15 or more (depending on the association).