It states it increases the chances of finding rare reagents when gathering.
Does this effect skinning, tailoring or disenchanting?
Or does it only affect mining and herbalism 
Discerning Eye specifically mentions gathering, so I believe Skinning qualifies but disenchanting does not. Rather, it increases Perception so what does Perception state? Gathering as well, so back to the first sentence.
If it involves the rare drops such as Dense Hide or Lustrous Scaled Hide, those would be consider what will be affected with improved chance by the racial ability because it affects Perception.
This too also affects some of the rarer things like knowledge point items because it is considered a rare item. Anything outside the basic reagents that are harvested by most reagants will usually be considered rare reagents. So usually this applies for gathering and any profession with their respective knowledge point items that can be found by different means.