how to check the database name that ActiveRecord uses

In Rails 4.2, you can do this:

ActiveRecord::Base.connection.current_database

You can also ask specific models for their database (since it's possible to use different databases per model):

User.connection.current_database

Rails.configuration.database_configuration

This will give you a hash table with the configurations for each of your environments. E.g. to get your development database name:

Rails.configuration.database_configuration["development"]["database"]

Since Rails 6.1 you must use ActiveRecord::Base.connection_db_config. So you can access the others class methods, like database().

db_config = ActiveRecord::Base.connection_db_config
print db_config.database
# main available methods: [:host, :schema_cache_path, :migrations_paths, :config, :database, :_database=, :checkout_timeout, :reaping_frequency, :idle_timeout, :replica?, :configuration_hash, :adapter, :pool]

An additional way to get more iformation is to use the database specific connection info methods. For example, if you are using postgresql, you can get details for the current database connection with:

ActiveRecord::Base.connection.raw_connection.conninfo_hash

This will give more connection details, not only those that differ from defaults.