Contributed modules
There are a couple of contributed modules which help you developing configuration.
The configuration inspector for Drupal 8
The configuration inspector uses Drupal 8's core built-in configuration system as well as a schema system to let you inspect configuration values and the use of schemas on top of them. This makes it possible to have a developer-focused overview of all your configuration values, and perform various testing and verification tasks on your configuration schemas. The following is a screenshot taken from https://drupal.org/project/config_inspector this module lists the available configuration data for user.settings:
