I wish people would stop recommending polygraphs. Polygraphs are notorious for false positives. Moreover, there are also easy ways to fool them. AND: people who give honest answers to the 'control' questions are more likely to fail the test than people who lie.
The polygraph is not a scientific instrument. It's used by government agencies as a screening tool to scare people into confessions and to say 'hey, it sucks that our employee was a spy, but we did polygraph him, so we're not to blame.'
Polygraphs don't measure lying and truth. They measure physiological arousal, and they're calibrated to the level of arousal measured when the subject answers a control question such as "have you ever told a lie" with a lie ("no"). That's it.