How is it that the Virgin Mary was able to understand without discernment that it really was the Archangel Gabriel appeared to her?

How is it that the Virgin Mary was able to understand without discernment that it really was the Archangel Gabriel appeared to her?

Very simple question with simple biblical answer.

Jesus in the Gospel of Matthew 5:8 says:

Blessed are the clean of heart, for they will see God.

Mary's Immaculate Heart can see the supernatural beings in their Heavenly forms including the Majesty of God, that's why Mary had revealed that the God of Abraham, God of Jacob, God of Isaac exist in the Holy Trinity at the mystery of Incarnation. When one has a pure or clean heart, the Angels can appear not obscure.It is an encounter between a pure spirit and the purest creature, the Masterpiece of God. And the main reason is, because all angels are subject to Mary, the Queen of all Angels, they must appear to their Queen truthfully as their sovereign.

Queen of Angels

How is it that the Virgin Mary was able to understand without discernment that it really was the Archangel Gabriel appeared to her?

Mary had the following to her advantage during the Annunciation:

• Mary was as the Angel Gabriel stated “Full of Grace”.
• She was the unspeakably overshadowed by the Holy Spirit (Luke 1:35).
• Mary was able to give the free assent of her faith by graces given to her by God.
• Mary was docile to God’s will.
• The Holy Spirit constantly guided and inspired the Holy Virgin Mary.

At her Annunciation the angel sent from Heaven greets Mary, not by her name, but by a title, saying, “Hail, Full of Grace.” [The Greek word used by Luke here], “kecharitomene, indicates that God has already graced Mary previous to this point, making her a vessel who ‘has been’ and ‘is now’ filled with divine life” (Ignatius Catholic Study Bible). As the Catechism of the Catholic Church states, “In fact, in order for Mary to be able to give the free assent of her faith to the announcement of her vocation, it was necessary that she be wholly borne by God’s grace” (no. 490). The sanctifying indwelling of the Holy Spirit was uniquely present to Mary Immaculate from the first moment of her life!

And then during her Annunciation, probably while she was only a teenager, and after having given her consent, the “unspeakable” shadow of the Holy Spirit overshadowed Mary (Luke 1:45), and she became so possessed by the Holy Spirit, so fully divinized by His union with her, that a child, a Divine child, was conceived within her womb.

Saint Luke then shows us that Mary is fully alive in the power of the Holy Spirit, for when Mary visits her cousin Elizabeth there is a veritable explosion of Holy Spirit power at the very sound of Mary’s voice! (see Luke 1:39-56). Everything in Mary’s life is taking place under the guidance and power of the Holy Spirit! - The Holy Virgin Mary was Completely Docile to the Holy Spirit

Mary having been Immaculately Conceived by a singular privilege by God made it that She lived her life in an extremely high level of union with God and thus revived graces necessary to her station.

Defined by Pope Pius IX in Ineffabilis Deus, 1854, it states that Mary, through God's grace, was conceived free from the stain of original sin through her role as the Mother of God:

We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance of her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Saviour of the human race, was preserved free from all stain of original sin, is a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful.

Writing material in the first century was very expensive and there was also a practical upper limit on the size of size of scrolls. Thus, there was more detail to accounts than what was recorded. Gabriel may have announced who he was, but the detail wasn't considered word recording. Even the Jerome Bible Commentary (Roman Catholic) does not address this issue. However, note this verse from a Roman Catholic translation.

There are also many other things that Jesus did, but if these were to be described individually, I do not think the whole world would contain the books that would be written. -- Confraternity of Christian Doctrine. Board of Trustees, Catholic Church. National Conference of Catholic Bishops, and United States Catholic Conference. Administrative Board. (1996). The New American Bible: translated from the original languages with critical use of all the ancient sources and the revised New Testament (John 21:25). Confraternity of Christian Doctrine.

For the individual receiving the apparition, the answer embedded in the question is indisputably all that the recipient needs :

... when you have a real apparition from Heaven you just now [sic] that it is true.

[Copied and pasted from the OP]

Others, who did not personally receive the communication, will have to make up their own minds, based on several factors, such as their own confidence in the integrity of the recipient and whether the whole matter is in accord with other precedents recorded in scripture.

As to Catholic oversight, who choose to adjudicate over such matters, there appears to be a Vatican document published in 1978 which judges of 'private revelations' regarding 'apparitions' according to three, or four, Criteria.

1. Criteria
2. Intervention
3. Competent Authority
4. Intervention by the CDF

Wikipedia Normae Congregationis

How is this question different than other angelic visitations to John the Baptist's father and to the shepherds in the field?

I think it is self evident that the veracity of the prophetic message was validated by subsequent events which occurred prior to when the narrative was written down by Luke. In all 3 instances, empirical verification of parts of the angelic messages were possible: Mary became pregnant while remaining virgin, Zacharias became mute and later could speak again, and the shepherds in the field found the baby in the manger. Because the messages were empirically validated by the recipients themselves, the recipients (Mary, Zacharias, shepherds) could then trust the non empirical parts of the message.

In the case of Mary, because 9 months later she really gave birth miraculously (empirically validated), then it makes sense that she could trust the rest of the non-empirical content of the visitation which must have included the angel's self-identification as Gabriel. She then told the story to Luke herself or told it to another eyewitness who could then be trusted with retelling the non-empirical portion of Mary's angelic visitation to Luke.

What differentiates Gabriel's visitation to Mary from non-Biblical apparitions is that there was an empirically verifiable component of the message which validated the whole message, which would have bolstered Mary's trust that the visitation was legitimately coming from God. While Mary had to have some initial trust, God did not ask Mary to have 100% blind trust, because the message came with empirically verifiable sign, just as Jesus's resurrection is the sign of the trustworthiness of Jesus's whole message as our Messiah.

Conclusion: The discernment didn't have to happen during the visitation, but during the long period while "Mary kept all these things in her heart and thought about them often" (Luke 2:19), which spanned the entire 33 year career of Jesus.