ActionScript for Flash MX: The Definitive Guide, 2nd Edition By Colin Moock
Chapter 18. ActionScript Language Reference
capabilities.isDebugger Property
Flash 6
indicates whether the Player is a debugging version
read/write
System.capabilities.isDebugger
Description
The Boolean isDebugger property indicates whether the Player is the release version (false) or the debugging version (true). The debugging version of the Player is used in conjunction with the Flash authoring tool to debug Flash movies. This property is not normally needed by Flash developers.
The corresponding server string for isDebugger is DEB, with possible values of t (true) and f (false). See capabilities.serverString for details.