You don't have to. That was just put in the article by way of explanation: even if there are no external forces, the axis will precess, because spacetime itself is twisting.
What you need in the real world is an experiment where all external forces have been accurately accounted for.
Or at least try to identify the major external forces. The rest could be combined as noise. The question is, in this case, how would anyone know that all relevent forces have been identified and isolated, free from dependence. How would anyone know that there isn't a rogue force out there as yet still unidentified?