I wrote this little tool to check websites for Google FLOC. I hope I have understood Google's description of the server opt-out correctly.

FlocTester.py

#! /bin/env python
import argparse, multiprocessing, urllib.request,validators

if __name__ == '__main__':
    multiprocessing.freeze_support()
    argsParser = argparse.ArgumentParser()

    argsParser.add_argument("-u","--url",help="the url of the website",default="https://www.github.com",type=str)
    args = argsParser.parse_args()
    print("testing: " + args.url)
    if validators.url(args.url):
        print("valid URL")
        response = urllib.request.urlopen(urllib.request.Request(args.url,headers={"User-Agent":""}))
        PermissionsPolicyHeader = response.getheader('Permissions-Policy')
        if PermissionsPolicyHeader:
            if "interest-cohort=()" in PermissionsPolicyHeader:
                print("this website does not use FLOC")
            else:
                print("this website maybe use FLOC")

            if "geolocation=()" in PermissionsPolicyHeader:
                print("this website does not use geolocation")
            else:
                print("this website maybe use geolocation")
        else:
            print("this website maybe use FLOC and geolocation")
    else:
        print("not an URL")