How to discern between good and evil apparitions?

A soul to whom God has granted such a favor may be unable to describe it, they have misgivings about it, and quite justifiably. So they have to proceed cautiously, and even to wait for some time to see what results these apparitions produce, and to observe gradually how much humility they leave in the soul and to what extent it is strengthened in virtue; if they come from the devil there will soon be signs of the fact, for he will be caught out in a thousand lies.

If the confessor is experienced, and has himself been granted such visions, it will not be long before he is able to form a judgment, for the account which the soul gives will at once show him whether they proceed from God or from the imagination or from the devil, especially if His Majesty has granted him the gift of discerning spirits. If he has this and is a learned man, he will be able to form an opinion perfectly well, even though he may be without experience.

The really essential thing, sisters, is that you should speak to your confessor very plainly and candidly - I do not mean here in confessing your sins, for of course you will do so then, but in describing your experiences in prayer.

St Teresa of Avila, The Interior Castle, Sixth Mansion, chapter 9