You can use the Shade Closed Loop diagnostic tool to detect closed loops formed by sketched entities. A closed loop is a chain of entities that form a section that you can use to create a solid extrusion. For 3D sketch geometry, only the closed loops formed by valid entities appear shaded.

If the sketch contains several closed loops that are internal and external to each other, the outermost loop is shaded, and the shading of the internal loops is alternated. For a sketch with multiple Sketcher groups, the criteria to identify a closed loop is applied to each group independently.

Shading of closed loops is displayed by default. To hide the shading for any new sketches during the session, perform one of these operations: