What is it about the melody of a song that makes it unique from the harmony?
To illustrate, if I hear a men's quartet sing a song I've never heard before in four part harmony where the melody is not necessarily the highest part and the singers make no effort to make the melody stand out through dynamics or other techniques, I am fairly certain that, with a little concentration, I would be able to pick out which notes belong to the melody and which notes are harmony.
There are many ways to describe the difference between melody and harmony subjectively. For instance, the melody is the "main part" or the "foundation" of the music and the harmony "complements" the melody.
There must be some objective differences between melody and harmony in music theory, right?