If two of more line features are selected, the Find Non-Connected Line Endpoints option will appear when right-clicking. Selecting this will display a dialog allowing you to either mark all line endpoints that do not connect to an endpoint (not an interior vertex) of another selected line or those line endpoints that don't connect and are within some distance of another selected line endpoint. This is a very useful feature for allowing easy identification and fixing of connectivity issues within loaded vector data.
Any line endpoints that do not connect based on the selected criteria will be displayed with a blue circle over that endpoint. Once you think you have fixed the connectivity issues at a location, simply re-run the command to find the non-connected line endpoints to update the display of the blue circles (they are not automatically updated as you join line endpoints).