there is a 'settle up' for the stimulus and if you are owed more (and that includes you didn't receive and you believe you should have), it will appear on Line 30 of Form 1040 as a tax credit which will increase your refund or reduce what you owe. 

just answer the questions as you go through the TT interview but the 'proof' will be Line 30. 

Just follow the interview ... everyone will see this screen in the Federal Review tab ...

The question about the amount you got in the advance stimulus payment doesn’t happen until the END of the interview.  By then the program has already calculated the credit and has included the amount in the refund meter you see at the top of the screen.   UNTIL you get to that screen where you can indicate how much you actually got in the ADVANCE payment the program cannot know what the correct amount really is.  So once you enter in that amount the program adjusts the credit accordingly depending on how much you already got and the refund meter will change appropriately.