The CDC publishes that data.[1][2] It runs 6 weeks behind, though.
[1] https://www.cdc.gov/coronavirus/2019-ncov/cases-updates/abou...
[1] https://www.cdc.gov/coronavirus/2019-ncov/cases-updates/abou...
The circles indicate nucleocapsid (i.e., infection only) while the squares indicate spike (i.e., infection or vaccination).
So, Puerto Rico is 70% infected and/or vaccinated (a square). Whereas NY is 13% infected (a circle).
There's also a blood bank antibody test. That's useful as a crosscheck. People who have blood drawn for medical purposes are in general sicker than average, and those who donate blood are healthier than average. So those set bounds on the measured values.