Apple - How can I tell if I'm viewing a secure page in Safari (6.0.2)?

On HTTPS pages Safari shows a small padlock.

(screen made with Safari 6.0.2)

You can see this a little easier in these larger snaps - one of a site where the SSL certificate was not trusted implicitly (signed or otherwise trusted by your Mac) and another where the certificate was valid and signed up the chain by the Mac and shown in green.

Some pages will display the lock icon during loading, then it will disappear. This is because there is some insecure content on the page, for example Google AdSense ads on a secure page can cause this.

As a follow up to the answer by @chrisg I recently noticed that some Safari Extensions will also hide the lock icon, presumably because they send or receive information over http. So if you are having problems when no site will show the padlock, lock through your Extensions...

(I know this question is a bit old, but I thought this might be valuable information)