But how do you know that the substance has changed if the accidents always remain the same?
They don't know and the early church viewed the Eucharist as symbolic, not the actual, literal flesh and blood of Christ.
In the medieval Roman church, the Eucharist literally gave life, and the threat of taking away the Eucharist to individual as well as entire nations was a means Rome used to keep them in submission to Rome.
ST. IGNATIUS OF ANTIOCH
St. Ignatius became the third bishop of Antioch, succeeding St. Evodius, who was the immediate successor of St. Peter. He heard St. John preach when he was a boy and knew St. Polycarp, Bishop of Smyrna. Seven of his letters written to various Christian communities have been preserved. Eventually, he received the martyr’s crown as he was thrown to wild beasts in the arena.
“Consider how contrary to the mind of God are the heterodox in regard to the grace of God which has come to us. They have no regard for charity, none for the widow, the orphan, the oppressed, none for the man in prison, the hungry or the thirsty. They abstain from the Eucharist and from prayer, because they do not admit that the Eucharist is the flesh of our Savior Jesus Christ, the flesh which suffered for our sins and which the Father, in His graciousness, raised from the dead.”
“Letter to the Smyrnaeans”, paragraph 6. circa 80-110 A.D.
“Come together in common, one and all without exception in charity, in one faith and in one Jesus Christ, who is of the race of David according to the flesh, the son of man, and the Son of God, so that with undivided mind you may obey the bishop and the priests, and break one Bread which is the medicine of immortality and the antidote against death, enabling us to live forever in Jesus Christ.”
-”Letter to the Ephesians”, paragraph 20, c. 80-110 A.D.
“I have no taste for the food that perishes nor for the pleasures of this life. I want the Bread of God which is the Flesh of Christ, who was the seed of David; and for drink I desire His Blood which is love that cannot be destroyed.”
-”Letter to the Romans”, paragraph 7, circa 80-110 A.D.
“Take care, then who belong to God and to Jesus Christ - they are with the bishop. And those who repent and come to the unity of the Church - they too shall be of God, and will be living according to Jesus Christ. Do not err, my brethren: if anyone follow a schismatic, he will not inherit the Kingdom of God. If any man walk about with strange doctrine, he cannot lie down with the passion. Take care, then, to use one Eucharist, so that whatever you do, you do according to God: for there is one Flesh of our Lord Jesus Christ, and one cup in the union of His Blood; one altar, as there is one bishop with the presbytery and my fellow servants, the deacons.”
-Epistle to the Philadelphians, 3:2-4:1, 110 A.D.