Awaiting for ur reply.
If you know in what text frame the XML loads, each TextFrame has a property 'overflows' -- "If true, the TextFrame has overset text."
I also found "XmlStory.overflows" ("If true, the XmlStory has overset text."), which might work as well, and "Story.overflows". I think you can simply find which text frame is displayed as the last one (the one with the red plus), by using their textContainers property: "Array of text frames or text paths" (those containing the story/xml story). XmlStory.textContainers[XmlStory.textContainers.length-1] will refer to the last text frame that is visible.
Personally never used these, but it might help you get started.