How can I see which sites have set the HSTS flag in my browser?

Chrome:

• Open Chrome
• Type chrome://net-internals/#hsts in the address bar of chrome
• Query domain: if it appears as a result, it is HSTS-enabled

Firefox:

• Open file explorer
• Copy and paste the following path into the address bar of your file explorer
  • On Windows: %APPDATA%\Mozilla\Firefox\Profiles\
  • On Linux: ~/.mozilla/firefox
  • On Mac: ~/Library/Application Support/Firefox/Profiles
• Double click the folder you see (if you have multiple Firefox profiles, there will be multiple folders)
• Open SiteSecurityServiceState.txt. This textfile contains sites that have enabled HSTS.

There is a Firefox plug-in called PinPatrol that lists all sites (preloaded and visited) known to have HSTS support.