As with the previous gospels, there are a number of specific and particular details in John attesting to its reliability.
The Sea of Galilee, for example, is described as having unstable and unpredictable weather. A storm could rise up, seemingly, from out of nowhere, making fishing a risky business. Modern meteorology verifies that the weather of the lake region is, in fact, unpredictable.
This implies that the text was written by someone who had actually been there, as people who live in a place long enough tend to be familiar with their region.