polygraph tests don't tell you if someone is lying, they simply measure a response to a question. The machine measures changes in heart rate, blood pressure, and breathing.
They also have machines to monitor changes in pupil dialation, which I believe to be more accurate.
If the examiner is skilled, you should be alright. He should have asked you several control questions like, have you ever stolen something from a friend?
They will use this to determine a "truthful" response or a "false" response.
