How would you prove employment at that job if it was many years ago and the employer is no longer in busiess?
The IRS keeps records of previous employment, as long as your employer filed w-4 form upon hiring you. Even if the workplace is no longer existent, the IRS will have these records.
You can start by going to this page (< href="http://www.irs.gov/localcontacts/index.html" title="IRS Local Offices">http://www.irs.gov/localcontacts/index.html) and searching for the local IRS office nearest to the place of prior employment.
On the other hand, if no w-4 was filed for this work, the IRS will have no record of it. If your employer did not file a w-4, then there is no guarantee that any record will still exist.