The same as you would for any other eyes.
If I said that I could see a space station through a telescope, what type of evidence would you expect me to provide?
Or not believe it.
Do you believe everything that people say? If I said that I saw the the eyes of the Holy Dragon and it allowed me to see magical kingdoms, would you believe me?
When have stories in books ever been evidence that someone existed?
Do you think Harry Potter is a real person?
Are you aware that humans are able to write stories in books about people who never existed?
If you claim it is a reality, then you should be able to produce evidence that it is a reality. If it only a belief, then it isn't a reality.
No, Harry Potter is fictional so he is not real, he was made to be fictional and for entertainment. The apologetics site, CARM, does a good apology confronting the position you hold with the dragon and Harry using Santa Claus:
First of all, Jesus is presented as an historical figure by reputable people in both secular and sacred historical writings. Santa Claus is simply presented as a fictional character.
Second, Jesus is presented as a real person who claimed to be divine and who performed miracles. These accounts are attested to by reputable witnesses and have been transmitted to us reliably. The New Testament documents are 99.5% textually pure. Santa Claus is intentionally and knowingly presented as a fictional character who lives at the North Pole.
Third, the intention of the Gospel writers was to convey the physical reality of Jesus to responsible adults whereas the accounts of Santa are intended to entertain the wild imaginations of children. This is why the vast majority of healthy, mentally competent adults do not believe in a real person known as Santa who can travel through air being pulled by several flying reindeer, who can carry in his sled enough presents for all the good children in the entire world, and who can descend and ascend through chimneys even though he is quite overweight.
Fourth, the writings concerning Jesus exhibit a historical, cultural, religious, and political context with verifiable names, events, and places being an integral part of the record of that context and reality. Santa Claus stories do not contain any such integral contextualization except to state that there is a North Pole and that there are cities and countries where Santa visits at night.
Fifth, the facts are that parents buy, wrap, and deliver presents to children, and we know of no documented occurrences where Santa Claus has been caught breaking and entering, tripping home alarm systems, caught on film, vanishing up a chimney, and riding a sleigh through the air pulled by flying reindeer. This latter point is worth a comment since we additionally have no evidence at all that reindeer can fly. This further adds to the irrationality of the Santa Claus story. Additionally, if a large sleigh (sufficient to carry millions of toys) approached the Washington D.C. area (surely there are at least some good children there), we would expect to hear of military fighter jets being scrambled to intercept the intruder. No records of this have yet surfaced.
Sixth, given that the Gospel accounts were written by individuals who knew Jesus personally (or were under the guidance of those who knew Him) and that the Gospels are historically accurate and superbly transmitted to us through the copying method, we can then assume at the very least that Jesus was an actual historical person. But, we have no hard evidence to establish the validity of Santa Claus. We have found no reindeer tracks on the roofs of millions of snow-covered homes on Christmas Eve. There are no video accounts of Santa roaming throughout peoples' homes. We know of no flying reindeer, and no one has yet established how Santa can live at the North Pole for hundreds of years without being detected--particularly in this technologically advanced culture. Add to that the lack of Santa Disciples going about the world risking their lives, being ridiculed by religious and political adversaries, writing inspirational text, performing miracles, etc., and you really don't have much evidence at all that Santa exists except in the minds of children.
Finally, it really comes down to whether or not either one can reasonably be proven to exist. Very few people deny the historic reality of Jesus. Though millions of children affirm the existence of Santa, we know well that the minds of children are not capable of differentiating between fantasy and reality--particularly when the parents they are trusting tell them Santa is real.
For an atheist to reject Jesus' existence based on arguments found against Santa Claus demonstrates the inability for the atheist to distinguish between historical, verifiable documents and known, constructed children's stories. Jesus was an actual historical figure. Santa, of course, is not.