So how do you get the visibility required to discern between front and back-end application execution issues? You have to get inside the user browsers, which most mainstream web APM vendors accomplish via a sort of page tagging technique much like you would do with Google Analytics, or other web analytics tools. While this technique is simple and works quite well for tracking in-browser execution, and while some top APM vendors will try to convince you that this is all that is required for comprehensive real user experience monitoring, be careful to note that this is only part of the story.
We recently developed a pretty detailed on whitepaper on this topic. If you're interested in learning how to update your application performance monitoring strategy for Web 2.0 and AJAX, you can read it here.