Finding All Insecure Content on a Secure Page
You can use SslCheck
It's a free online tool that crawls a website recursively (following all internal links) and scans for unsecure content - images, scripts and CSS.
(disclaimer: I'm one of the developers)
Try: www.WhyNoPadlock.com It will give you a report of all insecure content on any https web page.
Note, in recent versions of Chrome, these errors will be displayed in the Javascript Console.
e.g.
The page at https://mysecuresite.com displayed insecure content from http://unsecuresite.com/some.jpg.