Traditionally I use FireBug to debug my AJAX requests. It lets you examine both the contents of your request as well as the response that was sent back from the server. (it
If you are on a dev channel of Google Chrome:
http://www.chromium.org/getting-involved/dev-channel
...you should be able to right-click in the Developer Tools console, and click "Enable XMLHttpRequest logging".
Once it is enabled, you will see the XHR requests in the console, and will be able to click on them to take you to the resources panel, where you'll be able to see the content of an XHR.