Depends on whether or not the states check it for validity and my knowledge is so out of date that I can't say if they do.

You'd have to avoid any SSN which exists in an illegal range and you'd want the first three numbers to be consistent with where you claim to have applied for the SSN (usually the state where you were born - fewer questions). See also: http://ssa-custhelp.ssa.gov/app/answers/detail/a_id/425/~/de... and http://www.usgennet.org/usa/ne/topic/resources/basic/ssn.htm...

whose legit SSN? You are hiding your identity so why start with using your SSN, then what are you hiding?

For example, use any SSN that can be validated, whether in use or not (I think there is a formula to validate SSNs)

Social Security numbers are old-fashioned and don't have check digits like ISBNs. They are issued in patterns, but what you are trying to land on is a Social Security number that you can reliably use.