Your 2015 tax return shows your 2015 Adjusted Gross Income (AGI) If you filed a joint return then the AGI is the same for each of you.
Form 1040EZ line 4
Form 1040A line 21
Form 1040 line 37
If you are using the same TT account that you used last year, you should also be able to find this number in your Tax Timeline for 2015
You can get a free transcript of your past return from the IRS: