How do we know the Eucharist is the Body and Blood of Christ?
Because Jesus said so.
Scripture
From Genesis to Revelation, The Eucharist is an over-arching theme. God’s Truth is revealed in the details. For example the word Bethlehem translates to "House of Bread" and the infant Jesus was first placed in a feeding trough. Beautiful symbols foreshadowing the Eucharistic banquet. Reading scripture through the lens of the Eucharist changes everything.
History
From the very earliest days of the church, Jesus's followers held that the Eucharist was truly His Body and Blood. Even Protestant historian ND Kelly wrote “Eucharistic teaching, it should be understood at the outset, was in general unquestioningly realist, i.e., the consecrated bread and wine were taken to be, and were treated and designated as, the Savior’s body and blood” (Early Christian Doctrines, 440).
Science
Thomas the Apostle required tangible proof before he could believe in the resurrection. Jesus still provides us with miracles to bolster our faith.
What now?
Still have unanswered questions?
Rather watch than read?
The videos below give a wonderful introduction to the Eucharist.