This is the definition of Tradition from the Catechism of the Catholic Church
**TRADITION: **The living transmission of the message of the Gospel in the Church. The oral preaching of the Apostles, and the written message of salvation under the inspiration of the Holy Spirit (Bible), are conserved and handed on as the deposit of faith through the apostolic succession in the Church. Both the living Tradition and the written Scriptures have their common source in the revelation of God in Jesus Christ… The theological, liturgical, disciplinary, and devotional traditions of the local churches both contain and can be distinguished from this apostolic Tradition "
We know what’s in Tradition because Jesus promised his Church he would protect it from error. Individuals can err, but the magisterium in union with the pope cannot with regard to faith or morals. We know what is in Tradition because it was has always been taught.