Some sites that offer free Social Security Number traces are SSNValidator.com and SearchBug.com; however, searches regarding present employees can be freely validated through SSA.org, a government service. Read the guidance and conditions thoroughly before registering with the latter, as failing to use the service appropriately is a federal offense.
If the first three digits of the social security number are available, OmniTrace.com offer a full list of their corresponding states, which can help narrow down a search. Social security number searches outside of the SSA.org website are often limited in number.
If identity theft is suspected, guidance from the social security administration is to contact them if ever asked to provide social security numbers through unsolicited emails or phone calls.