The convention is that the north pole flips to the north due to the magnetic field of the earth. In other words: The only convention that we have is: earth's geographic north = earth's magnetic south and vice versa. So in order to correctly label your magnet, you have to figure out how it interacts with the magnetic field of the earth. Or, if you have another magnetic that is labeled correctly, you can check which end of your magnet sticks to that magnet. If it sticks to S, then that side is N etc.