"How would you rate yourself as an employee on a scale from 1-10?"
Now ask them for the supervisors name from the last employer - write down the name and ask how to spell the last name - then ask for that persons phone number.
"when I call _____ from your last job, how is he going to rank you"
If the answer is a 7 or lower - don't hire.
I also like "tell me about a time when you had to deal with a upset customer and what was the outcome"