If you were to attach anyone, no matter how virtuous, to a lie detector, you would eventually register a signal that suggested they might not be telling the truth. There is no such thing as a person who doesn't tell the occasional little white lie and such machines are notoriously unreliable. You can test this by finding the inventor of the lie detector itself and wiring them up. Ask them 'does this device really work?' Then draw your own conclusions! But none of this means that you can't trust the person that you need to trust.