Flash CS4 Professional.
bitmapItem.sourceFileExists
Read-only property; a Boolean value of true if the file that was imported to the Library still exists in the location from where it was imported; false otherwise.
Assuming the first item in the Library is a bitmap item, the following code displays "true" if the file that was imported into the Library still exists.
var libItem = fl.getDocumentDOM().library.items[0];
fl.trace("sourceFileExists = "+ libItem.sourceFileExists);bitmapItem.sourceFileIsCurrent, bitmapItem.sourceFilePath