But MTed, presumably SOME of the people who want to know if a tune is modal are (say) fiddlers, flute players, or unaccompanied singers. In that case, they are not using chords, and your definition has no meaning.

In fact, as greg stephens points out, it does not address the tune per se at all, but the harmonies or drones one chooses to add to it. As a singer, the drones vs. chords thing helps me not one bit.

You also complicate matters by then saying that the lack of chords is merely a consequence of a gapped scale. So is THAT your definition of modal, a gapped scale? If so, why not say that?   Of course, a gapped scale is an odd definition, since the primary named modes are no more or less gapped than most other scales in use in classical and pop music.

One problem, I think, is that maryrrf began with an odd question: "is it modal?" If we accept, as most musicologists would, that modes are just another name for scales, then all music is modal. The question is WHAT mode it is in, not "is it in a mode?" People than make up their own meaning for "modal" and use that. Many people use it just to mean "music not in the common scales of classical and pop music." So, to speak modally, "not Ionian or Aeolian, but in other modes." So you get your pentatonic, etc. Others use it to refer specifically to certain scales: Mixolydian, Lydian, Phrygian, etc. In the end, as M.Ted points out, there's no consistency in the way the term is used.

To answer another of Maryrrf's questions, yes, a skilled and highly literate musician who studies the modes can listen to a song and decide which mode it is in; but it is often easier to do so by transcribing the tune than by ear alone.