We walked the usual way (Leon to Oviedo) in 2012, and found the way mostly well marked. Not sure, but I think it would be possible to find markings walking the other way if you are using a good guidebook and paying close attention to turns. You'd want to be careful in some sections, especially on the first day between Oviedo and Mieres, and also between Pola de Lena and Pajares. The higher mountain stretch between Pajares and La Pola de Gordon I recall as well marked with bright, yellow-painted, posts that would be visible from either direction.
Buen camino.
Dan