Usually, it will state the line number for the dependent whose Social Security Number was already used or that needs to be corrected.
You would also want to verify that the rejection stated specifically that it was rejected due to the "dependent" by reviewing the rejection code and message emailed to you.
If there are two taxpayers listed on the return and there is an issue with the taxpayers Social Security Number, it will say if it is the first or second person. For example, "The name or Social Security number for the second taxpayer (spouse) doesn't match IRS records", this would be the second person on the return listed as the "spouse".
Rejection Codes: Social Security number and last name don't match IRS records
R0000-503-02 - Rejection code for taxpayer the 02 normally signifies the second person/spouse listed on the return.
R0000-504-01 - Rejection code for the dependent the 01 normally signifies the first dependent listed on the return.Enter your E-mail address. We'll send you an e-mail with instructions to reset your password.