Mark Open Polylines
The Mark Open Polylines 
tool finds
every polyline in the drawing that is not closed and shows it selected, with a spot
marker at each of its free end points. Open contours are a frequent problem in
sheet-metal work because a part outline must form a closed loop to be cut.
Using the tool
Select the Mark Open Polylines tool. All open polylines are immediately selected and their end points are marked. Inspect the highlighted polylines and close or repair them as needed — for example with the Join tool.
The markers and the selection are cleared when you switch to another tool. This tool only inspects the drawing; it does not change any geometry.