Check if database connection can be established

My application can be used with or without a database so I need to know if a database is correctly configured to show or hide functionalities which require a database.
How can I access to database config vars to check if they are filled?
Everything I found on the web requires to do a 

Code:

$this->load->database();

 so I can access to database vars from the 

Code:

$this->db

 object.
The problem is that if we do not have a database or if the database config file is not properly set, 

Code:

$this->load->database();

 will results in a PHP error:

Unable to connect to your database server using the provided settings.