The Capabilities Class in the flash.system package is perhaps a class that most people do not use. It is a very useful class that contains several useful properties and functions that can provide a world of information. I use the class regularly for finding the screen-resolution for my AIR apps using the screenResolutionX and screenResolutionY properties.
Here’s a small app using the class that detects the Flash Player Version and the player type – Debug or Release. This can be used as a FlashPlayer Version Checker.