how can i determine that my container is fill with another object ,e.g when i drag an object and drop into another container

If this is at runtime, check these two events out:

http://docs.unity3d.com/Documentation/ScriptReference/Collider.OnCollisionEnter.html

http://docs.unity3d.com/Documentation/ScriptReference/MonoBehaviour.OnTriggerEnter.html

Either of those should be relatively straightforward to implement and one of them might provide the functionality you need. The pages have script examples included as well.