Start by comparing the Form 1040 in the desktop and online tax returns line by line. You don't have to download the .tax2019 file from Online to see the Form 1040. (See below.) What is the first line on Form 1040 that is different (by more than a dollar or two)? What are the exact amounts on that line on the two returns? Once you know where the difference is, you can dig deeper, if necessary, to find the cause. Knowing where the difference is on Form 1040 will tell you where to look in the underlying forms, schedules, or worksheets.
In the desktop software you can view Form 1040 in forms mode, or "print" the tax return to create a PDF. For instructions to view your Form 1040 in TurboTax Online see the following FAQ.
How do I preview my TurboTax Online return before filing?
