You can home in on which side bearing is the rougher one by pushing the axle inward as you turn the axle. This is assuming that the bearing adjustment has the normal amount of freeplay that should just go away as a properly-adjusted quick-release skewer is applied.

I agree that a less than perfect cup/cone ball bearing might last years if it isn't yet showing signs of the raceway hardening flaking off to create pits.

I've set up axles with a file mark on the visible edge of an installed cone which corresponds to the location of any pit defect in the cone. The marking is then positioned upward when the wheel is installed, which takes load off of the pitted area on a cone.

Again, the axle cone adjustment must have freeplay before the axle-shortening tension of the quick-release skewer is applied.