Having read about the recent foot traverse of Antarctica by [Colin O'Brady](https://en.wikipedia.org/wiki/Colin_O'Brady) I was wondering which means the polar explorers of former, pre-GPS times used to determine that they had reached the North or South Pole. Obviously, you cannot use a compass since it would guide you to magnetic north or south and not to the pole itself. 

So what means did they use or (to make it a bit more open) could they have used?