How to check if HTTP requests are open in browser?

There is not a way to detect an open connection from JS unless you write a wrapper for XmlHttpRequest (or monkey patch it) that keeps track of opened connections.

Here's kidcapital's monkey patch, not sure if it's perfect, but it's a good start

  (function() {
    var oldOpen = XMLHttpRequest.prototype.open;
    window.openHTTPs = 0;
    XMLHttpRequest.prototype.open = function(method, url, async, user, pass) {
      window.openHTTPs++;
      this.addEventListener("readystatechange", function() {
          if(this.readyState == 4) {
            window.openHTTPs--;
          }
        }, false);
      oldOpen.call(this, method, url, async, user, pass);
    }
  })();

Note

This does not handle fetch or websockets but you could do something similar in those cases.

You can use FireBug for that.