Mediocrity knows nothing higher than itself, but talent instantly recognizes genius.What Doyle noted nearly a century ago is now known as the Dunning-Kruger effect. It shows that people who are incompetent in areas as widespread as humor, grammar and logic are often blissfully unaware of their own shortcomings. It also shows that incompetent people are often unable to recognize genuine competence in others.
-- Sir Arthur Conan Doyle, (Sherlock Holmes) Valley of Fear, 1915
I've talked before about second-order incompetence, and about how it relates to programming. Conventional wisdom among programmers seems to be that being aware of your own shortcomings is really a sign of competence. Knowing that there are things you don't know is a sign of a wise and experienced developer.
This isn't far off the mark, but according to follow-up studies by Dunning and Kruger, it's only a first small step. The skills required for competence in a particular area are the same skills necessary to recognize competence in yourself and in others. It still takes concentrated study in that area in order to raise your competence to the level necessary to be a good judge.