If you pick up a new client in 2021 or 2022, how will you know if they had this deferral?

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Best I can tell, there's no way to know.  I did find this section in the instructions for Schedule 3 Line 12e interesting:

"If you file Schedule H or Schedule SE, you can defer some of the
household employment and self-employment tax payments you may owe on
your 2020 tax return and pay them later
instead. However, you can't defer
amounts that you have already paid."

Does that mean they'll use any refund on the 1040 to offset the deferral?  What a mess!