This is one of the reasons for the recent stress on the Mishnah's statement, "Make for yourself a Rav." Since the Rav is neutral and uninvolved, he will certainly be able to give sound advice.
Even with this advice, however, a person might complain that he's unsure whether or not he chose a proper Rav. Here again, the Torah provides guidance, in a verse which also speaks of the pre-Messianic era (Malachi, 2:7), "...seek Torah from his mouth, because he is an angel of G-d." The Talmud explains, "If he resembles an angel of G-d, then `seek Torah from his mouth,' and if he does not, then don't."
But how can one tell if the Rav resembles an angel of G-d; one never even saw an angel of G-d! Here again, the Torah provides guidance, in the works of the Rambam, where he describes the lives of angels: "there is no eating or drinking...no jealousy, hatred or enmity."
Therefore, in order to tell whether or not someone is fit to be a Rav, one must see if he fits this description. Is his spiritual life governed without influence of physical factors (corresponding to "no eating or drinking")? Is he free of jealousy, hatred, etc.?
Of course, as always the Evil Inclination comes along with another objection – and one "according to the Torah" (since it likes to conceal its true motives in the holy garb of a "silk kapote"). "Isn't one of the signs of a true talmid chacham," claims the Evil Inclination, "that he is `vengeful like a serpent'?" According to this reasoning, peaceful behavior would not be a correct way of identifying a qualified "Rav"!
Fortunately, the Torah also answers this clearly. When is it proper for a talmid chacham to behave in this way? Only when someone has shamed him publicly, and a general insult to the Torah is involved. However, should he be insulted in private, the Torah requires the exact opposite response. In the words of the Rambam, the way of talmidei chachamim is to "listen to insult without answering back; and furthermore to forgive the person who uttered the insult."
Aside from these signs of a Rav, there is an obvious prerequisite: that the person has the signs indicative of a Jew in general. As the Talmud says, "This nation has three signs: they are merciful, bashful, and kind." Since these are called "signs," it is impossible that a person practice them only in private. To be considered a sign, the person must actually behave in these ways.