Very often the first sign is a model on its way to eternity, most cases of severe flutter result in crashes. On minor cases, you hear it as a usually rapid or statico sound like a flag flapping in a breeze or a rapid fire machine gun off in the distance. In two personal cases, a sound like a shotgun going off and the disappearance of the covering on the top half of a wing (landed without further damage that time), again a rapid flapping of the left half of a stab/elevator followed by it seperating from the rest of the airplane (again landed without further damage).