You could tell them just that; that you are looking to test 'their foxpro knowledge as well as the general logic and organizational skills' to determine the best candidate.

Then ask them how they would deal with an arrogant boss who him/herself
does not have the 'foxpro knowledge as well as the general logic and
organizational skills' to be a successfull programmer or even warrent
being in charge.

Also, you might ask what they think of companies that have high attrition
rates among programmers.