Are Canonizations Infallible?
If the Church declares someone a saint, how can we know with absolute certainty that they are in heaven?
The Church does not officially say that canonization is infallible, but theologians have always believed it is. The process of severe scrutiny sometimes takes years. And most importantly, asking God for a sure sign of blessedness (miracles) to show the Church the power of the Saint with God. Also, the nature of the declaration is another pointer since the Pope is officially decreeing that a specific person is in heaven. A decree to be believed and held by all Christians. It might be considered to be infallible.
As St. Thomas Aquinas noted:
Since the honor we pay the saints is in a certain sense a profession of faith, i.e., a belief in the glory of the saints, we must piously believe that in this matter also the judgment of the Church is not liable to error (Quodlib. IX, a. 16).
From New Advent:
These words of St. Thomas, as is evident from the authorities just cited, all favouring a positive infallibility, have been interpreted by his school in favour of papal infallibility in the matter of canonization, and this interpretation is supported by several other passages in the same Quodlibet. This infallibility, however according to the holy doctor, is only a point of pious belief.
Theologians generally agree as to the fact of papal infallibility in this matter of canonization, but disagree as to the quality of certitude due to a papal decree in such matter. In the opinion of some it is of faith (Arriaga, De fide, disp. 9, p. 5, no 27); others hold that to refuse assent to such a judgment of the Holy See would be both impious and rash, as Francisco Suárez (De fide, disp. 5 p. 8, no 8); many more (and this is the general view) hold such a pronouncement to be theologically certain, not being of Divine Faith as its purport has not been immediately revealed, nor of ecclesiastical Faith as having thus far not been defined by the Church.Catholic Encyclopaedia