Polygraph Tests are known more colloquially as "Lie detector tests". The theory behind these tests is that the human body reacts with certain levels of stress when giving dishonest answers to questions. The polygraph reads subtle changes in skin perspiration and heart rate to provide a drawing that can be read to determine whether lies are being told. Although polygraphs are highly technical to read accurately, once the basics are understood, it is possible to spot areas in which untruths are being told.
Ask the participant in the test 'base' questions, which have answers which are known to be true and false. For example, ask them to say "Yes" when asked if their address is "The White House". This will give you a reading as to how they react when lying. Repeat the step with an answer known to be true. These will serve as base readings.
Conduct the full interview, asking whichever questions are needed. It is possible to read the polygraph while the interview is being conducted. A large line drawn by the machine denotes a false answer, and a smaller line, or no line, denotes the truth.
Compare the potential false statement line with that of the known falsehood from the beginning. If you are unsure, then re-ask the question to gain another reading. Placing an interviewee under a great deal of stress may cause false negatives, so be sure to maintain a calming tone.
Use your judgement to state which areas of the interview were met with lies. Although larger lines do correlate with falseness, stressful memories or fear will also cause large lines, so compare the results with your memory of the interview. It may be best to take notes of the interview so as to compare observations on the interviewee with the the results. A polygraph will never give one hundred percent accuracy, but a professional judgement can be made.