Thanks for the question.
We know there are saints in Heaven because Jesus made the promise to save souls and bring them to Heaven. We even see evidence of that during the Early Church when the New Testament letters and books were being written. For example, in the book of Hebrews, Hebrews 12:1 says, we are surrounded by a cloud of witnesses, and in Revelation 11:12, "in the sight of their foes they went up to heaven in a cloud."
So, all throughout the Church's history, the faithful have recognized persons that lived such exemplary lives that God's grace and work was very evident in their lives. After their death, they were recognized as intercessors in cases of miraculous healing and other causes, and as a result there was a sort of Canon of Saints forming — without an official process of formal scrutiny.
Over time, the Church developed more rigorous standards by which to determine whether someone was worthy of this recognition, and today the formal process of Canonization is quite exacting.
There are many sources you can refer to for a more complete explanation of how the Church canonizes Saints today.