Excerpted from chapter 12 of How Do I Do That In Lightroom by Scott Kelby

In the Library module, first go to the Catalog panel in the left side Panels area and click on All Photographs, so we’re searching through all your Lightroom images. Then, press Command-L (PC: Ctrl-L) to bring up the Library Filter bar (it appears right above your thumbnails in Grid view) and turn on the Attributes filters. Now, if you look in the bottom-right corner of that bar, you’ll see the word “Kind” followed by three icons. Click the center one (as shown circled above) to display nothing but your virtual copies.