Reference: Paul Hebert, Fist Full of Talent.
No one is honest when there is a punishment for honesty.
Ask an employee if the incentive program drives her performance and she'll say no. Why? Because she doesn't want to admit (or believe) that she is influenced by a small reward. Then, ask she ever checks her score against her colleagues. Of course she does. They all do.
The lesson? Often, you can't ask people questions directly and get a straight answer. Instead, you have to probe for clues. When they can't see where the question is leading, they'll tell you the truth.
One of the best things to ask about are their very specific behaviours.