There's probably several ways we could go about this...
Archology: Archeological evidence has proven the Bible true time and time again. Just a few examples... King David was once thought to be a myth, like King Author, until a reference to the House of David was found. The Hittites were also treated as mythological until more than 10,000 clay cuneiform tablets confirmed they existed.
Prophecy: Biblical prophecy proves the Bible to be true. Just to name one, Daniel accurately predicts that the Messiah would be cut off (Jesus' crucifixion) and the city and sanctuary (Second Temple) would be destroyed, which happened in 70 AD (Dan 9:24-27).
Healing: There are plenty of healing testimonies out there. Here's one of a little boy who drowned and was diagnosed with irreversible brain damage. His parents were Bible-believing Christians and prayed for a miracle. Today, he's perfectly healthy and normal. I know it's true because he goes to my church, and I've heard his testimony and seen the pictures firsthand.
Miraculous!
Creation: Even nature will tell you there is a God. When you look at how finely tuned the universe and earth are for life and the complexity of DNA and humans, it all points to a creator. Science once believed the universe was eternal; now they believe it has a beginning, just as the Bible says. What is little known is that the Big Bang theory, the foundation of modern cosmology, was the work of a man who was not only a physicist but also a Catholic priest. He was only able to come to that conclusion because, unlike his peers, he saw no contradiction between science and the Bible. In fact, science was born out of the Christian West, and many of the early scientists were Christians.
Transformation: We can reasonably conclude it's true because it works and changes lives. I can think of no other "religion" where a person listens to a 30-minute sermon, gets saved, and his life becomes so radically changed afterwards. It's not an outwardly change but an inwardly change. They may have been some of the worst people before, but like the apostle Paul, they were changed in an instant. It is well known that it was Christianity that tamed the Vikings. If you know anything about the Vikings, that was a miracle in itself.