That's not true. Contemporary references to Jesus outside the New Testament include writings by Roman historians like Tacitus and Suetonias, as well as Jewish historian Josephus, who all mention Jesus and the early Christian movement within a few decades of Jesus' death which means they were all living when Jesus lived.
Aside from that I know someone who has in their possession used swaddling that Jesus wore in the manger.
All kidding aside, I would say the proof is in the secrets hidden and buried in scripture like treasure that Jesus revealed about the true subjects of Jewish law. No fictional character could possibly have ever done that.
Only a real person, a Jewish man enlightened by God, could have done that.