To determine if the object has left the screen boundaries, you can use Interact2D.projectCenter3D2D(); This will give you screen coordinates of the object's center. While it doesn't take the object's dimension into account, it should be sufficient in your case, if you roughly know an object's dimension on screen.
Or use Object3D.wasVisible(), which returns false if the object wasn't on screen in the last frame. This will work too, but it requires the object to be rendered offscreen at least once before it can be discarded.