Often the hard part is coming up with the right question.  Saw this on Michael Kellerman's Recruiter's Online:

ASK FOR PROOF OF SALARY

RECRUITER: What salary are you at now?

Most people will stretch the truth in response to this question. Even if its just by a few thousand dollars, candidates will always inflate their current salary.

There is a way to prevent this. Ask for proof.

If a bank is allowed to ask to see a pay slip to approve a credit card for $2,000, why cant you ask to see a pay slip when discussing a salary of $70,000?

Another way to phrase the question might be, If I were to ask for a payslip, what salary will it indicate you are currently on?.

While it might cause the candidate to become fidgety for a minute, or to break eye contact for a second, you are more likely to get a straight answer.

http://recruitloop.com//5-interview-questions-for-manager/