As many know, if you serve a page over https and the content loads resources (images, stylesheets, js, SWF objects, etc) over http, older versions of Internet Explorer will show the user a warning saying "This page contains both secure and non-secure items". This is discomforting to many non-technical users.

Usually, I can look at the HTML source and identify which item(s) are triggering this error. Sometimes a Flash object will load something else or some embedded javascript will put a new object in the DOM and trigger this.

What tools are good for quickly tracking down the source of the warning?