Jesus advises John to look at the evidence,

Matthew 11:2-11 NRSV
When John heard in prison what the Messiah was doing, he sent word by his disciples and said to him, “Are you the one who is to come, or are we to wait for another?” Jesus answered them, “Go and tell John what you hear and see: the blind receive their sight, the lame walk, the lepers are cleansed, the deaf hear, the dead are raised, and the poor have good news brought to them. And blessed is anyone who takes no offense at me.”
As they went away, Jesus began to speak to the crowds about John: “What did you go out into the wilderness to look at? A reed shaken by the wind? What then did you go out to see? Someone dressed in soft robes? Look, those who wear soft robes are in royal palaces. What then did you go out to see? A prophet? Yes, I tell you, and more than a prophet. This is the one about whom it is written, ‘See, I am sending my messenger ahead of you, who will prepare your way before you.’ Truly I tell you, among those born of women no one has arisen greater than John the Baptist; yet the least in the kingdom of heaven is greater than he.

The theme for the third Sunday in Advent is the forerunner of Jesus: John the Baptist.
The first thing to note is that John, now in prison, may be having doubts about his understanding of who Jesus is, for he sends a message to Jesus saying “Are you the one who is to come, or are we to wait for another?”
Here we see John doubting his own conviction. This should not disturb us.
To quote Joan Chittister ‎"Doubt is what leaves us open to truth, wherever it is, however difficult it may be to accept. Without doubt, life would simply be a series of packaged assumptions, none of them tested, none of them sure, and all of them belonging not to us, but to someone else whose truth we have made our own."
John, here, is asking a question. So should we. Jesus, in turn, responds by saying “Go and tell John what you hear and see.” In other words, he says look at the evidence. The blind receive their sight, the lame walk… and so on.
So, in this first paragraph, we have a paradigm, an example of how we might proceed. Doubt elicits a question, and the response to the question is to demand that we look at the evidence. Notwithstanding the nature of the evidence which some in the modern world will find difficult to receive, never the less there is a clear critical approach here. Doubt creates questions which require evidence in order to be satisfied. So much for the first paragraph.
In the second paragraph, Jesus asks the question about John:
“What did you go out into the wilderness to look at? A reed shaken by the wind? What did you go out to see? Someone dressed in soft robes?”
The implication is of course neither. People were not coming into the desert to look at the desert nor to find princes in their fine robes. No, they had gone to find a prophet, which is no more than somebody who speaks truth to a situation. Prophetes means ‘the one who appears on behalf of.’ and the Hebrew word ‘Nabi’ is ‘the one inspired to speak’. Both speak into contemporary situations. Prophesy is not primarily about prediction.
The message of John and Jesus is very simple: Repent. Change the way you look at the world. It seems to me that this repentance, this change, is a continuous process, because change in thinking will always follow changes in understanding. And changes in understanding will often be the result of some new piece of information, some previously unknown evidence.
What is it you hear? What is it you see? How does that change the way you understand the world?