If you mailed the federal amended tax return by certified mail through the US Post Office, they would have given you a return receipt when it was accepted by the IRS.
The IRS was not processing any type of paper tax return for several months due to the pandemic and limited staffing. They have only just recently started to process paper returns based on when they were received.
Go to this IRS website for the status of an amended tax return - https://www.irs.gov/filing/wheres-my-amended-return
