What form from 2016 jointly-filed return shows individual income?

Hi, appreciate any help on this question.

My wife and I filed jointly for 2016 tax year. 
She's a German citizen with a green card for the US. Her government wants to tax her on our income, though only I had income in 2016. But I have no idea how to verify that for the German government. Is there a form I can request from IRS, or find in my filed return, that shows who made income? We've already sent them a PDF of our return. They said there wasn't anything in it that verified she didn't make income. 
It's 800 Euros they want, so I'm willing to deal with this hassle or it'll never go away.

Thank you!