Check a function before doing anything?

Do you want to check if a function exists or if the result of your function exists ? I say that because your example looks like checking if $this->auth->is_login() == TRUE.
If you want to check if a method in a class exists, you can use method_exists() or is_callable().
Be careful : method_exists() will return true (or false) for a private/public/protected method whereas is_callable() returns true (or false) for only public method in a class and all method like __call.