As I said, you have to check for or Style Sheet -if a particular one is applied to every Running Head- or the box name -if ever one have been applied through AppleScript.
The kind of code would be "tell every paragraph whose style sheet is "Running Head"" or "tell every text box whose name is "Running Head"". (If you don't know if boxes have been named, just run a check code like "return every text box whose name is not """. If the result is empty then there is no box named. So you'll have to consider for the 1st option.)
Is this clear enough for you? Oh yes, in case you're really new at applescripting, mind you have to wrap the code here above into adequate tell blocks. Call again if needed!