The IRS will not tell you if someone else claimed your dependent. If you are claiming a dependent and your return is being rejected for duplicate use of their SSN, then the only way you can file is by mail. Print, sign and date your return in ink, and mail it in. The IRS will sort out the duplicate use of the SSN.
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.