I have always used this one.
Say you are running register one. Someone you know, but do not associate with often, comes in, says hi and takes a candy bar. Then proceeds to walk out the front door without paying. What do you do?
100% of the time people say tell the boss.
Next question: Same situation, but it is your significant other or a loved one. What would you do?
I would say about 85% of people say they would still tell their boss. Which tells me they have the ability to lie to my face.
The other 15% will say something along the lines of: I would let them walk out the door, pay the money, then confront them and ask them why they did it after my shift.
The second response shows honesty and the ability to think critical.