In John 13, how did Jesus show who would betray him?

Answer: He gave him a dipped morsel

Jesus said, "It is the one to whom I give this morsel after I have dipped it," and gave it to Judas son of Simon Iscariot.

“Jesus answered, “It is the one to whom I give this morsel after I have dipped it.” Then He dipped the morsel and gave it to Judas son of Simon Iscariot.”— John 13:26 (BSB)