MTed : I dont think the original question on this thread was about the old modes and their musicological history. It was about how to spot what is distictive about what is called a "modal tune" in American folk circles today. That is rather different.
    It would be simpler to say "flat sevenths" rather than a lot of the very interesting stuff you and Gargoyle have posted, which is fabulous stuff to have accessible: but not really on-message. "Modal". like "folk" can mean a lot of things.