Check it out:
http://www.mandalatv.net/2012/07/safari-6-0-activity-window/
From his site:
1) Open Safari > Preferences. It’ll bring up the Preferences window. Click on the “Advanced” tab and make sure “Show Developer menu in menu bar” is checked.
2) You’ll now see a “Develop” option in the menu. Open this up and choose “Show Page Resources”.
3) This will open up Safari’s new developer toolbar. Safari’s had this toolbar for quite some time, however, with Mountain Lion, it looks like Apple’s added some new features (perhaps most anticipated was a colored view source display). You’ll see a list of the page’s dependencies in the right column. Selecting them will display their contents in the center column.