[samizdis]

The EFF launched its site Am I FLoCed? [1] on April 9 for Chrome browser users to check whether they've been recruited/co-opted into Google's FLoC testing programme, started in March.

Not sure how it works, but the EFF doesn't seem totally confident that it can detect those affected: "This page will try to detect whether you've been made a guinea pig in Google's ad-tech experiment."

[1] https://amifloced.org/

[NilsIRL]

FLoC has a well defined API like (all) standards: https://wicg.github.io/floc/

There's no reason why the website would not be reliable at detecting whether FLoC is enabled on your browser.

You can also used https://floc.glitch.me/ which was linked in one of the blog posts from google.

[stefan_]

In the sense that you can call "side channel for ill-specified data Google wants to leak" an "API".