Using the capabilities object you can detect a whole load of stuff. The operating system, screen DPI, language, Flash player version and lots of other stuff.
Here is a list of the Capabilities object properties and what they are for :
hasAccessibility : determines if the player supports accessibility
hasAudio : if the player supports audio playback
hasAudioEncoder : determines if the system can encode audio
hasMP3 : if the player supports MP3 playback
hasVideoEncoder : if the system can encode video
input : the input device for the system (usually a mouse)
isDebugger : if the player is a debugging version
language : the current language setting for the system
manufacturer : the creator of the player
os : the operating system
pixelAspectRatio : width to height ratio of a screen pixel
screenColor : the color mode
screenDPI : the dots per inch supported by the screen
screenResolutionX : pixel width of the screen
screenResolutionY : pixel height of the screen
serverString : a string of URL encoded variables containing all of the capabilities property values
version : the version of the player