If you suspect that a woman you are or were involved with has committed paternity fraud, you should seek a DNA test immediately. (These can be ordered online, but a lawyer experienced in these matters may be able to advise you on which ones will stand up in court.) Parents looking for more information about child support should refer to the specific child support guidelines for their state.
Because the child’s mother may have intended the deception, it is highly unlikely you will be able to resolve the issue on your own. Make sure to speak with a qualified attorney experienced in resolving paternity fraud and obtaining reimbursement for past child support.