How can I detect 2 elements overlapping when dragging an element around?

How can I detect 2 elements overlapping when dragging an element around?

Hi, I would like to know how I can detect  when dragging around a element like a image or a div   over a image or another div how can I detect both that both are overlapping each other.

When this happens I want to fade in a window asking do you want to add this to this element?  with a yes or no button. The element that isn't being dragged is the one that I want to highlight so the user can easily understand that we are asking them the question do you want to add that dragged element to the one that is highlighted. 

If yes,  I need to then grab the html of the dragged element and then put it inside the div that wasn't dragged but our element was dragged too. 

How can I achieve this?