Q: Then you are saying that to judge another to be a genuine authority, we ourselves must be an equal authority.
A: Correct!
Q: Perhaps, but we can certainly recognize those who have superior knowledge to ourselves, and make them our gurus.
A: If you learn something from a person, something you are certain is true, then you can rightly assume the person in question knew it before you did. You may then infer that the person is probably in possession of further such truths; but remember this is only a statement of probability, and not certainty. Make this person your guru by all means, but it is wrong to assume the guru knows everything, or indeed that the guru knows any more than yourself.
Q: If we cannot be certain that another knows more than ourselves, then how can they be our guru?
A: A guru is a teacher of truth. If a person teaches truth, they are a guru, otherwise not.