How do you find your spouse's income from a previous year return if you filed jointly?

You need to manually add up their W2s and 1099 they got.   Why do you need it?   Do you need their AGI to efile?   On a Joint return there is only 1 AGI.   You enter the same AGI for each of you to efile.

Look at their W-2's and Form 1099's for the year or the IRS transcripts.  Separate income is not shown on a joint tax return.

To access your prior year online tax returns sign onto the TurboTax website with the User ID you used to create the account - https://myturbotax.intuit.com/

Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show.  Click on the Year and Click on Download/print return (PDF)

If you used the desktop CD/Download editions installed on your computer, the only copy of your tax data file and any PDF's will be on the computer where the return was created.  TurboTax does not store online any returns completed using the desktop editions.

Or go to this IRS website for free federal tax return transcripts - https://www.irs.gov/individuals/get-transcript