Polygraphs are flawed. People can learn to lie effectively with practice. Lookup the different kinds of physical responses that indicate lying and truth telling. Body language is the only real polygraph. Here's the big ones:
How To Tell If Someone Is Lying:
There are nine basic signs that liars exhibit. Not all of these signs may be witnessed, and not all mean that they are lying. They are just guidelines as to the body language that many liars display.
1) A change in the tone and pitch of the person's voice.
2) An increased or decreased rate of speech.
3) Stumbling over sentences, stopping to think during a sentence, increased use of 'um' and 'uh'.
4) A change in the amount of eye contact. Normal people make eye contact between one-fourth and one-half of the time that they are talking to you. Liars tend to make less eye contact.
5) Turning their shoulders away. (avoidance)
6) Looking very far to the left, right, up or down more than twice.
7) Touching their face or scratching their head.
8) Involuntary moving, jiggling, scratching, or other fidgeting of the legs or feat.
9) Crossing of the arms and/or legs.
I'd add fast blinking as another. Some can lie direct to other people's faces, but those subtle changes betray them.
Looking up to the right/left means they don't know. Looking past a person (eyes move to a spot behind but straight head) means they are conjuring up a lie. Looking down is lying. Now, I make less eye contact due to abuse. But, the other movements tattle on such people like me.
There is supposedly 16 different physical changes for men and 21 for women. Others, whining, anger, too much hand movement.
Look at voice changes. It's better to ask questions that require a simple yes or no answer (close-ended questions) than ones that require a paragraph/explanation (open-ended).
With those in mind, ask a question you know the answer to here and there. That physical appearance is your guide. Ask for no smoking or drinking. See if they violate the request with smoking. That's nerves. Good luck.http://how2dostuff.blogspot.com/2005/11/how-to-tell-if-someone-is-lying.html