The TextArea has a changing event, the event type is a TextOperationEvent.
The event has a property called operation. This operation can be many things, like an InsertTextOperation if they are typing, or a PasteOperation if they are pasting.
The operation also has a textflow property on it as well, but in this case the PasteOperation has a property called textScrap, and this has a textFlow of the text that is being pasted in. So you should have access to any styles coming in with the text.
Just a simple code snippet.
private function handler_textChange(event:TextOperationEvent):void
//see if action was cause of a paste
if(event.operation is PasteOperation)
//do what you need with the text flow object.
(event.operation as PasteOperation).textScrap.textFlow ........
Hope that helps out.