What's the level of the teaching that we can't be sure someone is in Hell? As far as I know, according to Catholic doctrine we can't be sure that any particular soul is in Hell. I understand the reasons (one can repent in the last moment, which is not observable by others), and I found some related doctrines (existence of Hell and whether it could be empty), but nothing indicating where in the levels of Catholic teaching the prohibition of proclaiming people to be condemned stands. I am almost sure it is not a dogma, but I've heard there are some canons against proclaiming that someone is in Hell, so it seems to be on some of the higher levels. If there really are such canons, I would like the answer to point to them.

EDIT: is it part of the infallible doctrine? If not, how strong is the view that calling someone condemned is a sin among Catholic moral theologians?
In Catholicism, we learn the teachings of Christ in the Mass as part of our Liturgy. We pray for all peoples, not just Catholics but all, that they find their way to fullness of the faith. Even at the highest positions of the church, or the lowest depending on your perspective, the authorities, those who watch over our souls (Heb 13:17) have a responsibility not to pronounce judgement, but to guide. There is only one who can pronounce judgement.

Be humbled in the sight of the Lord: and he will exalt you.
11 Detract not one another, my brethren. He that detracteth his brother, or he that judgeth his brother, detracteth the law and judgeth the law. But if thou judge the law, thou art not a doer of the law, but a judge.
12 There is one lawgiver and judge, that is able to destroy and to deliver.
13 But who art thou that judgest thy neighbour?
(James 4:10–13).

The Catechism of the Catholic Church defines presumption this way.

PRESUMPTION: An act or attitude opposed to the theological virtue of hope. Presumption can take the form of trust in self without recognizing that salvation comes from God, or of an over-confidence in divine mercy. (CCC Glossary)

Paul also makes his position very clear concerning even his own security.

LET a man so account of us as of the ministers of Christ and the dispensers of the mysteries of God.
Here now it is required among the dispensers that a man be found faithful.
But to me it is a very small thing to be judged by you or by man’s day. But neither do I judge my own self.
For I am not conscious to myself of anything. Yet am I not hereby justified: but he that judgeth me is the Lord. 1 Corinthians 4

This is a far cry from suggesting that he is justified once by the cross, he continues..

Therefore, judge not before the time: until the Lord come, who both will bring to light the hidden things of darkness and will make manifest the counsels of the hearts. And then shall EVERY man have praise from God.

As for dogma:

God, by His eternal resolve of Will, has predetermined certain men to eternal blessedness.
God, by an eternal resolve of His Will, predestines certain men, on account of their foreseen sins, to eternal rejection.
From the work of Dr. Ludwig Ott, Fundamentals of Catholic Dogma

Now, it is different to say that if you sin, and continue to sin, you are in danger of being separated from God's love. That is an example of teaching. Stating that a person is in hell or will go to hell, is pronouncing judgement, a thing reserved only to God. A priest can teach you what actions are condemnable and guide you to repentance; he cannot, however, condemn your soul.