I have set the property AllowEventOverlap to false , the property is preventing the overlapping but no (red) color is shown to alert the user how can I achieve his ?
Please make sure that you are using an up-to-date CSS theme. Older themes may not include the *_shadow_overlap CSS class that is used to highlight the forbidden position.
You can create a new theme in the theme designer:
http://themes.daypilot.org/