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.