How can we discern God's action in mystical phenomena?
It is not always easy; it is not always necessary.
Although they are authentic, phenomena can distract us from what is essential and slow our progress toward God. Too many people let themselves be hypnotized by these experiences. There are spiritual guides who too easily confound mystical phenomena and mystical life, as if they were necessarily identical.
The Church, moved by knowledge and inspired wisdom, does not pronounce itself lightly and rapidly on the authenticity of mystical phenomena; as a result, certain Christians become impatient. The pseudo-mystical life can look strangely similar to the real mystical life. A flaw in the doctrine or in the attitude, be it of a group or of a person, is not easily discernible, except by experienced and wise directors. The flaw becomes a crack and the fissured building finally crumbles. The individual goes astray and the group or community falls to pieces.
It is necessary to examine the phenomena and classify the occurrences. Are they of divine, satanic or human origin? Before attributing to them a supernatural origin we must see if they do not have a natural human source. Numerous people are fooled by their imagination, the prey of emotions and maybe the victims of neurosis, if not of psychosis. What about their emotional balance? Are they unstable?
I will take advantage of your question to slip in a recommendation. Beware of those who wish to steer you away from the Church, in different directions, under the pretext that they have received a flash of spiritual inspiration. Jesus says: "And if anyone says to you then, 'Look, here is the Christ' or, 'Look, He is there', do not believe it; for false Christs and false prophets will arise and produce signs and portents to deceive the elect, if that were possible. You therefore must be on your guard. I have forewarned you of everything" (Mk 13: 21-22).
How can we discern God's action? As Jean Guitton mentions, it will be impossible at times to identify with certainty the origin of some or other phenomenon, of this or that vision. However, there are criteria of discernment, no matter how hard they are to apply.
First, there is true charity. A real mystical phenomenon does not hinder charity but prompts the soul to love God and neighbor more humbly and deeply.
All mystical phenomena that would reject Christ's and the Church's teaching should be rejected. All excessive, ridiculous and improper phenomena would not be of divine origin, asserts the likable Saint Francis of Sales, who wrote so well on devout life. The Evil One shows such a lack of balance.
Does the mystical phenomenon carry good fruits? Does it lead the soul to live a better Christian life?
Beware of spontaneous fruits that rot quickly. Appearances can be deceiving. Good fruit do not ripen in an instant. Discernment can take a certain amount of time.
Finally, does the mystical phenomenon produce the fruits of the Holy Spirit mentioned by Saint Paul in his letter to the Galatians: love, joy, patience, kindness, goodness, trustfulness, gentleness and self-control (Ga 5: 22)? Such fruits may be accompanied by suffering and persecution.
We can discern the authenticity of a religious phenomenon using the criteria of charity, sound doctrine, good works and the fruits of the Spirit.