I saved my entire return but several critical forms don't save. How do I know which forms were submitted when some of them don't save?

Which forms didn't you get?  Some don't need to go.  

How to get or print a copy after filing
https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-get-a-copy-of-a-return-i-filed-this-year-in-turbotax-online/01/25932

Here's an alternate way to print by opening the filed return back up to print at the Print Center, where you can also get the worksheets.

You have to have your return open on the screen to get to Tax Tools and print.

Select Tax Home in the left menu.
Scroll down to Your tax returns & documents.
Select 2020, and then select Add a State (you're not actually adding a state, this just gets you back into your return).

After the return is open, click in the left menu column on TAX TOOLS, then PRINT CENTER.

Then choose Print, save, view this year's return.

The next screen should offer some options: Just my tax returns or include government worksheets (optional), or include government and TurboTax worksheets (optional.)

There's no way.

You are paying for TurboTax to do it correctly and submit all the forms that need to be attached to your form 1040.

If some form is missing, you will hear from the IRS about it. On very rare occasions, it's been known to happen.

I know how to save, print, and submit forms--that's not the issue here.

When you select "Save required forms," the program is not saving all required forms; 3 required forms were missing from my saved 'required' documents.  As the program didn't select all the required forms to save, I'm concerned the program isn't selecting all required forms for submittal to the IRS (assuming the same 'required forms' algorithm is used, and why would it not be?).   You have no way to know which forms the program selects when it submits to the IRS because you don't get a copy.  You should be given the choice to be cc'd on the IRS submittal.