How can I found out if my kids was clamed by someone else?

This is a tax question not a child raising question so its been moved from the Children forum. The reason we have all these forums is so people can find similar questions and maybe find the answer already. Since this question has been asked many times, you would have found that this is an issue due to e-filing. What I assume happened is you tried to e-file and it was rejected because someone had already filed claiming your children.

In this case, you need to file manually including proof that you are legally the one to claim your children. The IRS will then investigate. If this other person claimed them illegally, the IRS will go after them for the additional taxes owed and penalties. However, they may never inform you of who it was.