What shows that my iPhone is waterproof?

Nothing … because it’s at best only water-resistant.

The water-resistance specs are listed for each model here:

Identify your iPhone model - Apple Support

Here’s additional information to that already provided.

Water and other liquid damage to iPhone or iPod isn't covered by warranty - Apple Support

Regret late reply; you might also find this user tip informative:

https://discussions.apple.com/docs/DOC-250002708