Is it that obvious?????

This week we learned about rhetoric and the functions rhetoric holds. While constructing this post, I wondered about rhetorical questions...they are asked but the person asking already knows the answer. So with lie-detector tests, do they give you the test KNOWING that you're lying and that they already know the truth??? Or is it working in your favor and you know you're telling the truth. This reminded me of the movie Meet The Parents, in the movie the boyfriend (played by Ben Stiller) is hooked up to  a lie detector machine and is administered the test by Robert De Niro. I couldn't help but wonder, why was Ben Stiller character given the test by Robert De Niro, even though Robert De Niro character is a CIA agent. He should know these answers already, but he stills asks them just so he can catch Ben Stiller character in a lie; and the hence proves that he's unfit to marry his daughter.