Flash MX 2004.
Library.itemExists(namePath)
stringSpecifies the name of the item. If the item is in a folder, specify its name and path using slash notation.
True if the specified item exists in the library; false otherwise.
booleanMethod; 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"));