Pretty sure they decided via death differentials using 1917 data vs 1918 and 1919.
Also funeral candles (like during the middle ages) for Catholic countries.
There are different ways the CFR has been estimated for the 1917-1919 pandemic. I’ve always had questions about it, because the data is contradictory.
I found an interesting account from London in the 1600s during one of the Black Plague epidemics. A man had kept a diary and recorded his experiences. The trajectory of that epidemic is similar in some ways to what we are facing now.