How do we Know the Shroud of Turin is Real?

Because it says so.

"Jesus, Nazarene"
http://news.aol.com/article/vatican-researcher-sees-writing-on/704189?icid=mainmaindl1link1http%3A%2F%2Fnews.aol.com%2Farticle%2Fvatican-researcher-sees-writing-on%2F704189