Find the instruction letter at the front of the PDF you saved or printed when you filed the 2018 tax return with TurboTax.
You access that prior year TurboTax return this way:
1. Sign in, make sure you're using the same TurboTax account (with the same username) as in prior years.
2. Select Documents from the navigation when it appears.
3. Use the drop-down menu at the top of the screen to choose the tax year you want and then select Download PDF.
Also, you can check your bank statements from last year for a payment to the U.S. government.
[Edited 04/04/2020|6:50am PST]