If you're using NGUI properly for your events, this is done for you already and you don't have to do anything. The first camera to see something (GUI, if its depth is higher) will intercept the event.
If you have your own Input-reading implementation on the game camera, then it's more difficult as you need to do your own checks. On the PC its simple enough -- UICamera.hoveredObject, but on a touch based device there can be a few touches, and it becomes more complicated.