John asked Jesus who the traitor was. Jesus tells him that it is the one to whom he dips the bread and gives it to him. Judas was the only other person (other than John) within reach. Jesus dips the bread and hands it to Judas, clearly identifying Judas as the traitor. However, John is the only person who heard and understood the meaning of this act. In the next verse, Jesus tells Judas to do what he intended to do and to do it quickly.

20 Now when evening came, Jesus was reclining at the table with the twelve disciples. 21 As they were eating, He said, “Truly I say to you that one of you will betray Me.” 22 Being deeply grieved, they each one began to say to Him, “Surely not I, Lord?” 23 And He answered, “He who dipped his hand with Me in the bowl is the one who will betray Me. 24 The Son of Man is to go, just as it is written of Him; but woe to that man by whom the Son of Man is betrayed! It would have been good for that man if he had not been born.” 25 And Judas, who was betraying Him, said, “Surely it is not I, Rabbi?” Jesus *said to him, “You have said it yourself.”

CROSS REF: Matthew 26:20-25

When evening came, Jesus was reclining at the table with the Twelve. 21 And while they were eating, he said, “Truly I tell you, one of you will betray me.”22 They were very sad and began to say to him one after the other, “Surely you don’t mean me, Lord?” 23 Jesus replied, “The one who has dipped his hand into the bowl with me will betray me. 24 The Son of Man will go just as it is written about him. But woe to that man who betrays the Son of Man! It would be better for him if he had not been born.” 25 Then Judas, the one who would betray him, said, “Surely you don’t mean me, Rabbi?” Jesus answered, “You have said so.”

Jesus tells John that the traitor is the person to whom he gives the "morsel" (a portion broken off a larger piece [of bread]. This was the beginning of the Passover meal. According to custom, the host gives the first piece of the meal to the person sitting in the place of preeminence. This was Judas.

But we have direct testimony, that, about the time of Christ,5697 ‘the sop’5698 which was handed round consisted of these things wrapped together: flesh of the Paschal Lamb, a piece of unleavened bread, and bitter herbs.5699 This, we believe, was ‘the sop,’ which Jesus, having dipped it for him in the dish, handed first to Judas, as occupying the first and chief place at Table. (Alfred Edersheim, The Live and Times of Jesus the Messiah)