You can look at your 2015 return. Or do you not know if you filed a 2015 return at all??
If you didn't print out//archive a copy of your 2015 return for your records (btw, which you should always do), and if you used Free Edition for 2015, you'd have to pay to unlock the return. We can tell you how to do that.
Or you can download a free 2015 tax return transcript at this IRS website:
https://support.turbotax.intuit.com/contact
If you didn't prepare a 2015 return at all, you cannot file 2015 income with 2016 TurboTax. You can prepare a 2015 return by purchasing the 2015 desktop software. It cannot be efiled; it has to be printed, signed, and mailed.
If you DID file a 2015 return, but left out some W-2's, then you would need to prepare an amended return (Form 1040X) then print, sign, and mail it.
