It is best to start with John 20:30-31. Here are the words:
"Jesus performed many other signs in the presence of his disciples, which are not recorded in this book. But these are written that you may believe that Jesus is the Messiah, the Son of God, and that by believing you may have life in his name."
Three points are in order to understand these verses. First, according to the theology of John, Jesus did many miracles to show that he was the son of God and the long awaited messiah. This is why modern scholars speak of the signs of Jesus in the first part of the gospel. For example, Jesus' first miracle of turning water into wine was called a "sign" by the gospel of John. This sign is followed by many others such as healings of sicknesses and even the resurrection of Lazarus.
Second, John's gospel is also intentionally making reference to the Old Testament. He is showing that Jesus is the fulfilment of the prophecies of the Old Testament. In other words, the signs are specifically related to theme of fulfillment. For example, the miracles of turning water into wine is spoken of in many places in the Old Testament. When the messiah comes the hills will drip with new wine (Amos) and a person will be able to wash their clothing in the blood of grapes (Gen. 49). There will be abundance.
Third, from the perspective of the New Testament, the other miracles and signs of Jesus can be read in the other gospels. There are four of them and they give slight variations of Jesus' ministry.
All of these can be seen a evidence, according to the gospel writer of John.