How to detect insecure content on an https web page

Discover what is triggering the: "This page has insecure content" warning message
Having recently installed a secure certificate for ssl / https on a number of sites, I had a number of problems detecting where the insecure content was coming from. Viewing the page source showed only secure https links and none of the scripts or css files appeared to contain insecure content.

Eventually Google Chrome came to the rescue:


 * Right click over https page and select "Inspect Element"
 * Select "Console"
 * Select "All"
 * If there is insecure content, the warning will contain the source of the error.