The title bar of a material or map can quickly tell you whether it is being shown in the viewport and if there are instances of the same map/material in other material editor views.
When Show Map is on, a red flag appears in the Navigator and in the material/map's title bar.
Similarly, when if an instance of a map/material is in multiple views in the Slate Material Editor, it is shown with a grey flag.
Navigator display of a material with Show Map In Viewport turned on
Material/Map Browser entry for a material with Show Map In Viewport turned on