There are a few ways that you can tell if St. John is real. The first is by looking at the historical evidence. There are many documents and records that date back to the early Christians that mention St. John. Additionally, there are many paintings and sculptures from the Middle Ages and Renaissance that depict him.
Another way to tell if St. John is real is by looking at the theological evidence. The Bible mentions him several times, including in the book of Revelation where he is said to be one of the witnesses of Christ’s return. Furthermore, church tradition has always held him in high esteem as one of the most important figures in Christianity.
Finally, you can also look at the personal evidence. This includes things like miracles and healings that have been attributed to him over the centuries. Many people have had their lives changed by encountering his relics or visiting his shrine in Ephesus, Turkey. All of this points to the fact that St. John was a very real person who had a profound impact on history and culture