When going to Jerusalem, John describes the journey as ‘going up.’ The terrain surrounding Jerusalem is hilly, and the city is, in fact, built on top of a hill. The only way someone could know something that specific is if they interviewed someone from that part of the world (as Luke did in his gospel) or was personally familiar with the region (as John would have been).
Keep in mind, this book was written long before the internet; people couldn’t just look up information on a region. Also, this was centuries before the encyclopedia; another valuable tool of research.