How do we know Jesus wasn't speaking in a parable or something when He said, "This is my body"? - Matt & Jason

Do you realize that more hundreds of scholars have tried to pull that statement apart and tell us that it means one thing or another?  So, your question about how we know we can be sure that Jesus meant it literally is an excellent one!  Although many others don't understand what Jesus was saying, the Catholic Church is very positive about the statement being literal and meaning exactly what Jesus says.

We have a couple of things which allow us to be certain that He really meant what He said about us eating His body.  (By the way, we call the miracle which changes the bread to Jesus' true body and the wine into His true blood  "Transubstantiation.") 

First off, when Jesus taught, he often explained what he meant to the disciples.   If people didn't 'get it,' or if they 'got it wrong,' He clarified what he meant.  When He spoke about eating His flesh it was scandalous to many of the followers.   This idea of "gnawing" or "munching" on someone's flesh and blood was abhorrent to the Jews.  When He spoke of it, He didn't clarify.  He allowed the people to think it was what it is.

That being known, let's take a look at the conversation in John 6:515-69 ... Jesus says, "I am the living bread that came down from heaven; whoever eats this bread will live forever; and the bread that I will give is my flesh for the life of the world."  The Jews then quarreled among themselves saying, "How can this man give us his flesh to eat?"  ... Just a little bit later they say, "This ... is hard.  Who can accept it?"  and many left and returned to their former way of life and no longer accompanied him.  

Jesus didn't stop and say, "Wait, I meant that metaphorically!  Come back!"  Instead, ...

He looked at the remaining disciples and asked them, "Do you also want to leave?"  and Simon Peter said, "Master, to whom shall we go?  You have the words of eternal life."   (from the New American Translation of the Bible)

So, when Jesus said the words about eating His flesh at the Last Supper, they were not new to Jesus' followers.  They had already discussed and argued about what Jesus meant.  They knew He meant what He was saying.  And they believed.

Later, in First Corinthians, "St. Paul said that whoever receives communion unworthily is guilty of the body and the blood of the Lord."  (1 Cor 11:27)  In that culture, to be 'guilty of another's body and blood' is to be guilty of murdering them.  How could Paul say that someone was guilty of killing Jesus because he ate bread that symbolized Him?  Paul [too] knew that the Eucharist was not merely a symbol but actually the body, blood, soul, and divinity of Jesus."  (Did Jesus have a last Name?  and 199 other questions from Catholic Teenagers  by Matthew Pinto & Jason Evert, (c) 2005 Ascension Press, west Chester, PA  p139)

The Church Fathers strongly affirmed that Christ's body and blood becomes present in the sacrament of Eucharist.  The Council of Trent summarizes it too.   It is a beautiful  mystery of our faith which has been part of our history from the very beginning.  

Thanks for the question.  I hope this helps.