“Who having heard, was troubled at his saying, and thought within herself what manner of salutation this should be” (Lk. 1:29).
In this beautiful mystery of the Annunciation, remembered at every Ave Maria, we have from Our Lady herself an important key to the question of private revelations, even for her own apparitions. This key is the virtue of prudence, which always stands in a middle between too much and too little in everything.
Here we see Our Lady troubled by the angelic apparition and laudatory greeting, and we see her thinking within herself. The rest of the story shows how she then discerned that this angel really came from God, and at the end how she consented to his heavenly request: Fiat mihi secundum verbum tuum.
It is the Church that examines and discerns between true apparitions of Our Lady, of Our Lord and of the Saints, and false ones. It is the Church that tells us, based on the classic rules of discernment of spirits, such as those of St. Ignatius, if we can believe or if we must reject a particular apparition. Faith is always given to God through the Church: “He who hears you, hears me!”
The major problem we face today in this field is basically ignorance both of the nature of these supernatural manifestations and of the rules of prudence. The essential principles of discernment are contained in the Spiritual Exercises of St. Ignatius, nn. 328-336. I highly recommend the commentary of these by Rev. Fr. L. Barrielle, SSPX, published by Angelus Press [Rules for Discerning the Spirits]. I suggest also the Jesuit Scaramelli’s Discernment of Spirits (available only in second hand editions, in French, Italian or Spanish…). He expounds on the Ignatian rules and helps souls to discern, directly or indirectly, whether these supernatural manifestations come from God or from the devil. “By their fruits you shall know them.”