The United States government offers a free Social Security verification service for employers, located on the Social Security Administration website. This service is the only free official verifier available, as of 2015; all other services are either fee-based, not affiliated with the SSA or both.
The Social Security Administration's free verification service is for employer use only and is solely for checking the Social Security numbers of current or former employees. In addition, it is meant to be used for tax-filing purposes only.
The SSA also offers the Consent Based Social Security Number Verification Service for more general use; however, this service costs a fee.
Websites such as SSN-Check.org promise to verify Social Security numbers for no charge, but they are not official or even necessarily up-to-date. As of November of 2015, SSN-Check.org states that its database contains numbers issued up until June 25, 2011, the date the SSA began a randomization policy for new numbers. People born after this date are not in the SSN-check.org database.