Unfortunately, there is no way of knowing or finding out one of your dependents had been claimed on another return without first filing your return. At this point, the IRS would reject the return using rejection code R0000-507-01. This could have been intentional or by mistake.
When you get a rejection like this, the IRS is telling you that you will not be able to e-file your return because the SSN in question has already been used. The only option you have at this point is to paper-file your tax return. Just print, sign, date, and mail in your tax return.
After filing your paper-return, you'll eventually receive a letter from the IRS, stating that your child was claimed on another return. It will tell you that if you made a mistake to file an amended return and if it was not a mistake not to do anything. The other party will get the same letter you did. If one of you doesn't file an amended return, un-claiming the child, the next letter, from the IRS, will require you to provide proof. You will want to ensure you reply in a timely manner.
