The US CDC does this by re-using random anonymous blood samples drawn for other purposes, and by using test data from blood banks. The current data collected is for the presence of antibodies indicating people who've been infected. Not those vaccinated; they can distinguish that. This is cheap to measure, but runs about 6 weeks behind. Data here.[1] US average from late June is about 22% of the population. That's roughly how many people actually had the disease.

That data should show the Delta variant surge as the data comes in.

I still can't find a useful number for people with antibodies prevalent because either infected or vaccinated, although that is apparently tested.

[1] https://covid.cdc.gov/covid-data-tracker/#national-lab