There's No Official Way to "Find Out Who"
The IRS protects tax information very carefully. As a result, there's no legal or official way to find out who claimed your child on his or her tax return. In most cases, you'll never know that someone else claimed your child, unless you get a notice that your return is rejected because someone has already claimed your dependent. Two people can't claim the same dependent, so eventually, you'll receive this notice if you and someone else both claim your child.
Who Knows the Child's Personal Information?
One way you can make a guess at who else has claimed your child is by considering who else knows your child's personal information. In order to claim a child as a dependent, the claimant would need to know your child's name, social security number and date of birth. It may be an ex-spouse or someone who had access to your home and personal records, such as a babysitter or relative.
File Your Taxes Claiming Your Child
When someone else claims your child on their taxes, you only have two ways to resolve the issue -- file a Form 3949A: Information Referral, reporting an alleged violation of income tax law, or simply file your taxes claiming your child. A Form 3949A requires you to know who claimed your child. If you dont know who claimed your child, the best way to resolve the issue is to simply file your taxes and list your child, which will cause the IRS to open an investigation. You may need to file your taxes by mail, as electronic filing may cause a rejection of your return due to duplicate dependent claims.
Provide Proof of Eligibility or Relevant Forms
When two people claim the same dependent on their taxes, it triggers an audit or investigation by the IRS, whereby both claimants must provide proof of their right to claim the dependent. The residency test states that the dependent must reside with the claimant for more than half the year, and meeting this criteria may be enough to resolve the claim. If you're not the custodial parent, you may need to provide relevant forms, such as Form 8332, which is what a custodial parent uses to waive his right to claim a child as a dependent.