Assuming you’re using it (and if you’re not, you should be), open up Firebug and click on the "Debugger" tab. In the "Scripts" drop down at the bottom, select the script ending in com_sun_faces_ajax.js. Here are some useful lines in this script1:
Line 440: By inspecting the variable xml, you can see the response from the server.
Line 453: You can see the id of each component that is to be rendered.
Line 454: You can see, in the "content" variable on the right, the actual content that will be rendered.
There are obviously countless more interesting lines, but these are the only ones Ed discussed specifically, and should be enough to show those interested in such things where to start looking. Hope that helps.
While I’m on the subject, my boss was able to get jsf-extensions working in his project, so we’ll now be able to get a better feel for both jsf-extensions and Ajax4jsf.
1 In the interests of full disclosure and fairness to Ed, the descriptions of the lines you see are mostly direct quotes of him, with minor changes to make it more readable in this context. :)