lie detector 
noun
An instrument for determining whether a person is telling the truth by testing for physiological changes considered to be associated with lying. The results of lie-detector tests are generally not accepted for judicial purposes. Compare with polygraph.This, to Lily at least, sounded like a good enough answer, one that would definitely qualify under the lie-detector test as the truth....
- While NSA officials call the lie detector test a useful tool, it is clearly not perfect.
- Jack questions Greg at every turn, even going so far as to subject him to a lie-detector test and monitoring his every move with hidden surveillance cameras.