My wife and I are separated and when she did her taxes, she claimed both my children. Now when she received her taxes, she did not receive everything that she was suppose to get back because someone had claimed my son on their taxes. So she came straight to me and asked me if I had claimed him. How do I find out if someone she knows claimed my son on their taxes?

You can't find out if someone else has claimed your son on their taxes - individual tax information is confidential. If your wife is entitled to claim him, then she should do so. However, if someone else has already claimed him the IRS will not accept an e-file return from her (the computer sees the discrepancy and automatically rejects it). So she should file a paper return, and claim her son. The IRS will investigate and make a determination as to who the rightful person is. It would be helpful for her to include a signed statement with her return stating that she is the custodial parent of the child.