How can I find out how much my dtr dad got back for her on his taxes

There seems to be 2 questions here. The first in your title asks how to find out who claimed her. The only way you can even know if someone else claimed her is if the IRS informs you of that. And then they won't tell you who.

The second question in the body seems to imply that you know the father claimed her, but want to know what benefit he received. The only way to be sure would be to process his taxes with and without claiming her.

How can I find out how much my dtr dad got back for her on his earned income credit

How can I find out how much my dtr dad got back for her on his earned income credit

First, please don't start multiple threads for the same issue. I've merged your threads. Any further followups should be added as an Answer to this thread.

You would need to know the details of his income to calculate the credit (see IRS Publication 596).

But if he was not eligible to claim her, then you file your own return claiming her and let the IRS straighten it out.