If you have not filed the 2018 tax return and are accessing the 2018 online program -
You can view your Form 1040 at any time. Click on Tax Tools on the left side of the screen. Click on Tools. Click on View Tax Summary. The 2018 AGI will be shown in the Tax Summary.
If you have filed your 2018 tax return -
To access your current or prior year online tax returns sign onto the TurboTax website with the userID 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)
The 2018 AGI is on the Form 1040 Line 7