I don't think I can fully address your question (especially the 'prove' part), but: 1) Why not install Privacy Badger? ( https://www.eff.org/privacybadger http

I don't think I can fully address your question (especially the 'prove' part), but: 1) Why not install Privacy Badger? ( https://www.eff.org/privacybadger https://www.eff.org/privacybadger ) It watches which sites javascript files are loaded from, and if you go to three domains and get the same script, and it seems to be sending back information, it'll flag it and block it. 2) There is the possibility of seeing everything going from your browser to a remote computer. There are two ways. a) Use the developer tools. In many browsers, you can bring up dev tools (possibly by hitting F12), bring up the 'Network' tab, and hit the 'Preserve Log' button. Everything that goes between your browser and the internet is recorded. You could dive into every javascript file requested, see which data and URLs are requested, etc. In most browsers, you can save this data to a .har file for later analysis, but it will contain cookies, etc, and allow someone else to spoof you. b) similarly, you could set up a proxy (Charles Proxy, Proxie.app, mitmproxy, etc), to proxy HTTP traffic, but you'd also need to go to extra effort to proxy HTTPS traffic. Then, with your browser (or OS) set up to use the proxy, you can obtain the same sort of data as in (a). With this information, you could see which sites you are requesting data from, what the data is, and, I suppose, analyze the sources to find out what it is doing.