Did Mary know she was sinless, according to Roman Catholic teaching?

There are ways in which she can know (not "feel") has grace, although not with certainty (aside from a direct revelation).

In "Whether man can know that he has grace?" (Summa Theologica I-II q. 112 a. 5), St. Thomas Aquinas explains the ways the reality of grace can or cannot be known in one's soul:

There are three ways of knowing a thing:

1. by revelation, and thus anyone may know that he has grace, for God by a special privilege reveals this at times to some, in order that the joy of safety may begin in them even in this life, and that they may carry on toilsome works with greater trust and greater energy, and may bear the evils of this present life, as when it was said to Paul (2 Cor. 12:9): "My grace is sufficient for thee."

2. a man may, of himself, know something, and with certainty; and in this way no one can know that he has grace. For certitude about a thing can only be had when we may judge of it by its proper principle. Thus it is by undemonstrable universal principles that certitude is obtained concerning demonstrative conclusions. Now no one can know he has the knowledge of a conclusion if he does not know its principle. But the principle of grace and its object is God, Who by reason of His very excellence is unknown to us, according to Job 36:26: "Behold God is great, exceeding our knowledge." And hence His presence in us and His absence cannot be known with certainty, according to Job 9:11: "If He come to me, I shall not see Him; if He depart I shall not understand." And hence man cannot judge with certainty that he has grace, according to 1 Cor. 4:3,4: "But neither do I judge my own self … but He that judgeth me is the Lord."

3. things are known conjecturally by signs; and thus anyone may know he has grace, when he is conscious of delighting in God, and of despising worldly things, and inasmuch as a man is not conscious of any mortal sin. And thus it is written (Apoc. 2:17): "To him that overcometh I will give the hidden manna … which no man knoweth, but he that receiveth it," because whoever receives it knows, by experiencing a certain sweetness, which he who does not receive it, does not experience. Yet this knowledge is imperfect; hence the Apostle says (1 Cor. 4:4): "I am not conscious to myself of anything, yet am I not hereby justified," since, according to Ps. 18:13: "Who can understand sins? From my secret ones cleanse me, O Lord, and from those of others spare Thy servant."

^{(taken from this answer)}

This applies to the Blessed Virgin, who was a human being and knew according to the human manner of knowing.

Did Mary realize that she was sinless before the Annunciation?

No. If I cannot remember my sins, the only reasonable conclusion is that I forgot my sins, or that I was ignorant of having sinned, not that I was without sin. The same is true of Mary. She could not have known that her life so far had been free of sin, even if she could not remember any sins. (Furthermore, the doctrine that Mary was without sin applies to Mary's entire life, including the unknown future.)

So it would certainly not have been obvious that she was free of sin, and she would not have expected that God had selected her for the purpose in question. That was the very point of the greeting and the Annunciation: to inform Mary of something that she could not have known without revelation.

Did others realize that Mary was sinless?

It is impossible for others to realize this based on a person's actions. To have never sinned is something far beyond having lived a righteous life. Many sins are internal and therefore hidden, and many actions that seem sinful might be good. Those who knew Mary would have thought very well of her, but without divine revelation they could not have considered her to be without sin.

Why did Mary react as she did to the greeting?

• The greeting was spoken by Gabriel, an angel. Clearly, it was very serious.

• Gabriel calls her by the name "kecharitoméne", which means "full of grace" (Redemptoris Mater 8), and not by her proper earthly name, "Mary". Naming a person in this way is common even today (Smith, Baker, Sawyer...), and it was certainly common in the Bible (Abram, Abraham, Sarah, Jacob...). However, Mary is troubled by this particular name. She understands the language, so she knows what was meant. She did not ask for clarification of why she was being called by this name (as she did when the Angel revealed that she would conceive, though she was and planned to remain a virgin). Still, the name did weigh very heavily upon her.

• Mary had great humility. When Catholics say the Rosary with the Joyful Mysteries in mind, when the Annunciation is remembered, the prayer is "for the love of humility"). When a humble person is told that she is favored, she typically reacts with genuine surprise and perhaps confusion, because she has not until that point been imagining herself as favored. Most people might be very happy to be told that they are favored by anyone, but Mary just begins to think deeply about what God's message means.

Did it never occur to Mary, at any point in her life, that she was free of sin?

She could only come to have such a belief as a direct result of divinely-revealed knowledge, which may have included the revelation that she was the Mother of God (she was very much aware of this). Even then, she would have been aware of such knowledge only after the Annunciation (see the first question). Furthermore, there is no Church teaching (that I have been able to find) that suggests that the thought "I am sinless" had occurred to Mary. She does not dwell on attempting to recall past sins, and instead seems to spend much of her time contemplating what God is telling her.

There is no specific doctrine for this, but I think it can be answered logically with the help of scripture verses and with the doctrine of Immaculate Conception which teaches that Mary was sinless and remained so always.

The question whether Mary knew that she was sinless is hard to know because the question seeks the answer from the perception of person who is defined as sinless. Nevertheless it is possible to infer that she knew about it but possibly didn't know the specific purpose for which it was meant other than just obeying God's command to be so.

Mary was conceived immaculately which meant that at the time of conception she was preserved from the stains of Original Sin. As a consequence of this, the state of original sanctity, innocence, and justice, as opposed to original sin, was conferred upon her, by which gift every stain and fault, all depraved emotions, passions, and debilities, essentially pertaining to original sin, were excluded. But she was not made exempt from the temporal penalties of Adam — from sorrow, bodily infirmities, and death.

With this gift Mary would have no inclination to sin. Being unstained with the original sin of man means that she can also perfectly choose not to sin. She had a perfect will to not sin. A person selected by God would not choose to sin as that doesn't make sense.

Whether she knew that she was sinless:

We can all look back on our own lives and think of at least one sin we committed. In view of her immaculate conception as stated above, Mary couldn't have. Therefore she must have known she was sinless. But assuming our self to be sinless as compared to others and declaring oneself as saint is in itself a sin. So Mary possibly was aware that she was sinless but was humble enough before the Lord and other people in not thinking herself sinless because that was for God to judge. As Jesus said about two Pharisee in Luke 18:14, praying in two different way: one boasting about his righteousness and other accepting that he is a sinner.

Possibly she was not aware of the purpose of her holiness and just thought it to be an obedience to God. She must have been perplexed thinking, why specially angel Gabriel has to come visiting her and declare:

Luke 1:28 And coming in, he said to her, “Greetings, favored one! The Lord is with you.” 1:29 But she was greatly troubled by his words and began to wonder about the meaning of this greeting.

Lord can remain only with sinless beings. She knew that she was sinless but she was thinking why God has to send His special angel to her. As she was not knowing the purpose of her sinlessness, she was surprised with this special appearance of angel Gabriel. Gabriel understood her confusion in mind and declares:

Luke 1:30 So the angel said to her, “Do not be afraid, Mary, for you have found favor with God! 1:31 Listen: You will become pregnant and give birth to a son, and you will name him Jesus. 1:32 He will be great, and will be called the Son of the Most High, and the Lord God will give him the throne of his father David.

And Mary's response was that of blind obedience:another sign of her sinlessness.

In a "Short Catechism on Mary" French Cardinal Charles Journet wrote:

Q. How do we know the Virgin Mary never committed any sin

A. If the had committed even the slightest sin, she could not truly be called full of grace, neither would she have been the Worthy Mother of God

So, in one sense, if she pondered what the Angel called her, "full of grace" (in Latin at least), which she did. Then she would have known then and in reflecting on that she could truly say that "All generations will call me blessed".

But, as to her Immaculate Conception. She, being in time, would have had a hard time grasping her redemption which, although it had already occurred in eternity, still required a sacrifice on the Cross.

Q. Did the Blessed Virgin herself achieve redemption through the prayer of Jesus on the Cross?

A. Yes. It is because of the prayer that Jesus would later make on the Cross that God in advance preserved the Blessed Virgin from the original stain.

So, I'm sure that blows your mind. I certainly wish the good cardinal would have given a bit more context to that, but you wanted something official from the Church and I'll leave it there.