How can I Find out If some else claimed my child's social security number on there taxes?

If two people claim the same SS number for a dependent the IRS will investigate the discrepancy - that's how you'll find out. For confidentiality reasons you won't be told who has tried to claim your child (although in most cases it's pretty obvious). Unfortunately for the 2nd person who files with the same SS number for a dependednt - if the 2nd person tries to file electronically it will be rejected, as the IRS computers will catch the "mistake." The 2nd person is forced to file a paper return, and the IRS will investigate to see which of the two parties is entitled to claim the child as a dependent.