How do we know Jesus is real?
Some of the answer to the question "Besides the Bible, how do we know God exists?" might be helpful in thinking about this, but I’ll say a couple of more things.
Every day we make decisions based on things that we haven’t seen with our own eyes, but based on what others have seen and experienced and we believe them. Sometimes our own eyes would even lead us to disbelieve, but we still depend on what others have discovered and taught us. ie we can’t see that air contains the oxygen that we need to breathe, but we are convinced that it’s true.
Similarly when it comes to finding out about Jesus, we depend on people who were there at the time, to tell us what they saw and heard. The New Testament documents are far and away the most reliable and best attested ancient documents we have. There’s simply no room in the science or history to say they’ve been changed over the years, or they were made up in the first place. Whenever professors of ancient literature or history study them, when lawyers try and disprove them, they keep being shown to be entirely reliable. Archaeologists are constantly uncovering new ancient parts of the Bible, and they match what we read in our Bibles today.