Some Catholics are able to tell the voice of Christ and His Saints from phony revelations.
Jesus has a distinct way of speaking: He is fatherly, gentle yet strong, majestic yet sorrowful, divine and human - in short, His manner of speaking bespeaks of His Perfection and Character.
Mary has a distinct way of speaking: She is motherly, sweet, gentle, kind almost to a fault, yet queenly, commanding, motivational - in short, her manner of speaking bespeaks of her perfection and character.
All the saints have their own distinct way of speaking. Some you can tell from their writings and spiritualities, others from their prayers and deeds.
The Devil can masquerade as Jesus, Mary and the saints, even making the illusions of resurrections and other miracles - and though he can fake Humility, he cannot fake Obedience. He can fake Charity to a point, in as far as he cannot fake Obedience.
But when you know the voice of Christ and His Saints, you can know the real thing from the fake attempts - to a point. You can tell from brevity, manner, words used, personality - in short, familiarity with real private revelations - but you cannot be sure, absolutely, without the Church's approval.
Fatima, Lourdes, Laus, Rue du Bac, Akita, Medjugorje, Amsterdam, Bayside, and other apparitions, whether approved or unapproved, have shown that knowing the voices of God and His Saints is not enough to discern: It is a help, but it is not a sign of spiritual mastery.