You can see which fonts are blocked by StopTheFonts by using Safari's built-in web inspector. First, you need to enable the Develop menu in Safari's main menu. You can do that by opening the Advanced pane in Safari's Preferences/Settings window and checking "Show Develop menu in menu bar".
You should now see the Safari web inspector, with its Console tab selected. Enter "content blocker" in the search field to filter for messages from content blockers such as StopTheFonts.
Now whenever you load a web page (for example,
https://www.espn.com) in the Safari window, you'll see which web font URLs were blocked. The file extensions
ttf are for Web Open Font Format and Open Type fonts.