Before the centralization and standardization of canonization processes by the Holy See in the Middle Ages, Saints were most often “canonized” by popular acclamation, veneration within the religious orders, or proclamation by local bishop. There was no set, official process, per se, so that many Saints of the first millennium of Christianity have never been formally canonized!
The Archangel Michael (as also the Archangels Gabriel and Raphael) has always been recognized as a Saint, first, because “saint” means “holy one”, and we know from Scriptue that he is one of the holy angels, and second, because veneration as a Saint also means recognition of that person’s presence with God in the glory of Heaven - which we also know to be true of Michael, by means of the Bible. (It would thus not be inappropriate also to say, “Saint Guardian Angel”!) Incidentally, we do indeed know of St. Michael’s earthly activities, also through Scripture.