tell application "Adobe InDesign CS4"
tell document 1
return every text frame whose overflows is true
You can try this i am using JS Code:
var myDoc = app.activeDocument;
var myTF = myDoc.textFrames;
You don't have to check every text frame. Only check the final frame of each separate story
Why "the fastest way"? There may be a couple of different ways of checking, but surely the difference between them is minimal?
Just a word of warning:
The overset property is not foolproof. You can get false negatives -- at least when footnotes and tables are involved...