Similar to the way that you can call connection on a model, you can make a call to connection on ActiveRecord::Base.
ActiveRecord::Base.connection_config
Look at the docs for ActiveRecord::Base, as there are other methods that allow you to get/set attributes about the connection.
Update
The connection_config became deprecated and will be removed from Rails 6.2. connection_db_config should be used instead:
ActiveRecord::Base.connection_db_config