Kenichi Suzuki, one of my favorite SEOs from Japan, posted a screen shot on Google+ of a search tool filter to filter results by private content shared with me.
Here is his screen shot:
I personally do not see this feature because I am a Google Apps user, but most Gmail users should be able to see it by conducting a search and selecting "search tools" and clicking on the "private" option in filters.
This type of filter will show you content...
