How to detect if the user clicked the "back" button

try:

window.onbeforeunload = function (evt) {
  var message = 'Are you sure you want to leave?';
  if (typeof evt == 'undefined') {
    evt = window.event;
  }
  if (evt) {
    evt.returnValue = message;
  }
  return message;
}

You generally can't (browser security restriction). You can tell if the user navigates away from the page (onbeforeunload, onunload fire) but you can't tell where they went unless you've set up your page to allow it.

HTML5 introduces the HTML5 History API; in conforming browsers, the onpopstate event will fire if the user navigates back to an earlier "page" on your site.