I ask because I haven't yet heard of an existing antibody test for COVID-19.

I think they are just appearing nowish.

https://www.sciencenews.org/article/covid-19-coronavirus-pan...

There are many, especially in other countries. There are even some that one can purchase in the US, but it's not clear when (or if!) the FDA will approve these. See their Emergency Use Authorization FAQ page [1], where they list dozens of tests that are presumed to be available in the US.

There's some discussion in NY State about NYC hospitals having developed a test that's currently in use.

1. https://www.fda.gov/medical-devices/emergency-situations-med... ; look for "What serology tests are being offered under the policy outlined in Section IV.D of the Policy for Diagnostic Tests for Coronavirus Disease-2019?"