Airspace classes are 3D volumes. They are shown in sectional charts, but only in 2D. This makes them a bit confusing to understand at first.
You asked where on a chart does it tell what an airspace is. This is done by outlining particular airspaces using a specific color-coding. The outlines are made up of a combination of either magenta, cyan, or gray, and they are either solid, dashed, or shaded. Ideally you should memorize this, but as a last resort you can look them up in the charts's legend.