In the "How do I" sample project, there is a project "Determine where the user clicked in world coordinate". You can see more tricks related to spatial references on the map in a project in our Code Community.
You can see in the project "Latitude Longitude Coordinate" code.thinkgeo.com/projects/show/graticule some techniques to offer more spatial clues to the user navigating the map. We show how to display lines of latitudes (parallels) and lines of longitude (meridians). We also show how to get the latitude and longitude from the mouse pointer.