Debtor working off the books

I have a New York small claims judgment against an individual who works at a small retail establishment. When I had the Sheriff serve an income execution upon the debtor's employer, the employer responded that the debtor doesn't work there. I have personal knowledge that the debtor indeed works there "off the books", and probably paid in cash. My questions are:
1. What evidence do I need for the court and/or sheriff to recognize that the employer is not being truthful?
2. How do I obtain this evidence?
3. How do I proceed with this evidence to have the income execution enforced?
Thank you

[JETX]

Subpoena the employer in to court for a '3rd party debtor exam'. Put him under oath and ask away. Penalty of perjury will sometimes 'jog' the memory.

And depending on the type of work....
call the employer during work hours and ask for the debtor by name. Or you might try videotaping the debtor at work. Be creative.