If you filed with online TurboTax last year, it should have transferred into your online 2016 return, if you are in the same account.

Otherwise you can look at your 2015 return.  Is the 2015 return in the Tax Timeline of the same account that you are using for 2016?  If not, we can tell you how to troubleshoot multiple accounts to find it.

Also in the 2015 Tax Timeline,  under "Some things you can do" there should be a link for "View adjusted gross income (AGI.)"

Or you may be able to download a 2015 return transcript from the IRS at the link below.  If you don't qualify to download one, you can order one to come by postal mail in 5-10 calendar days.
http://www.irs.gov/Individuals/Get-Transcript

Also see this FAQ:

@King-david 

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