Flash CS4 Professional.
videoItem.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 that the first item in the Library is a video 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);videoItem.sourceFileIsCurrent, videoItem.sourceFilePath