If you know approximately where the aircraft is and if there is an airport nearby, you can view flight traffic around that airport. Use the airport code under "Airport Info" on left column and click on "View Activity." Use the map popout (orange box on lower right corner of map) and all the planes in the area will show up. Blue planes indicate traffic headed to/from the specified airport; green planes indicate air traffic in the area. If you mouse over a plane, it will show you which flight it is; click on a plane to see the flight details.