St. Joseph of Optina
From the life of St. Joseph of Optina.
The Elder [Ambrose], and Father Joseph were asked the same question about how to find a clairvoyant person, "one meets a man, apparently clairvoyant, and yet it feels there is something wrong, how do you know whether his clairvoyance is from God?" Both ascetics said the same: "Recognize these people by humility. Because the enemy can give clairvoyance to a person, but he never gives humility - it burns himself".