Similar to what Ashkan suggested you can use Camera.ScreenToWorldPoint to test that your screen corners would be within your viewable world space when the user attempts to move the camera.
This means you have to somehow define the bounds of your world, whether by finding the min and max location of all objects in your scene or by manually storing them.
Personally I would manually create an invisible box in each scene that encompasses the viewable world space and test against it.
