http://www.ssa.gov/history/ssn/geocard.html
I’m not sure how they proved his SSN, but SSNs are based upon the state they were applied for in.
SS #’s are on income tax forms. Politicians are required to turn those forms over under the law.
Anyone with a computer and $20 can get information regarding employment, address, SS#, criminal records, alias’, etc.. There are a number of sites used by investigators and private companies. Obama uses a SS # which matches his home in chicago. It is the same number he filed taxes with. He is an identity thief.