Multiple droppables: How to detect the droppable, which did not accept the draggable?

Multiple droppables: How to detect the droppable, which did not accept the draggable?

I'm not familiar with jquery ui in depth and didn't find a solution for the following simple setting:
I have a gallery with different kinds of waste and two trash cans, one for paper and one for 'bio'. When the user drags #waste_1 and drops it on #paper everything is ok, because .paper is accepted in #paper.
In case that the user drags #waste_2 and drops it on #paper it should revert and #paper should shake. Revert works fine but I didn't find a way to detect the droppable when the waste is not accepted.
The structure (simplified): 
  1. <ul class="waste"> <li id="waste_1" class="paper"></li> <li id="waste_2" class="bio"></li> </ul> <div class="trashcan"> <div id="bio" class="bio"></div> <div id="paper" class="paper"></div> </div>

In other words: I need a way to detect the droppable, where the dropped waste 'reverted from'. I hope my question is understandable and I would be grateful for any advice.