Flash MX 2004.
library.itemExists(namePath)
namePath A string that specifies the name of the item. If the item is in a folder, specify its name and path using slash notation.
A Boolean value: true if the specified item exists in the library; false otherwise.
Method; checks to see if a specified item exists in the library.
The following example displays true or false in a dialog box, depending on whether the item Symbol_1 exists in the
Folder_1 library folder:
alert(fl.getDocumentDOM().library.itemExists('Folder_1/Symbol_1'));