Remember that many pieces are multi-movement or divided into 3 or 4 smaller sections. There is generally a noticeable break between movements and a distinct change in musical mood or character. Generally if the tune is heard prominently, it is important. The more repetitions, the more important it is. A composer will choose the instruments carefully and manipulate these choices much like a painter chooses and manipulates various colors and textures.
Most concerts consist of works from a variety of historical eras and composers.
Each work will have a distinct character and shape. Try to describe those things which makes each work different from the others.