There is not a way to check that before you file. The IRS will not tell you if someone else claimed your dependent. If you are claiming a dependent(s) and your e-file is rejected for duplicate use of the Social Security number, then you can print, sign and mail your return. The IRS will process your return claiming your children, and then will sort out who could claim them. If someone who was not supposed to claim your child did so, they will face penalties and repayment of the child-related credits they received.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.
Federal and state returns must be in separate envelopes and they are mailed to different addresses. Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**