Similar to action hooks, information about commonly used filter hooks can be found on the WordPress Codex (https://codex.wordpress.org/Plugin_API/Action_Reference) or on sites that provide raw function lists (for example, http://hookr.io).

It is also possible to learn about filter hooks by searching for occurrences of the apply_filters function in the WordPress code. As can be seen in the following code, this function has a variable number of arguments, with the first one being the name of the filter hook, the second representing the value that the registered function will be able to modify, and the remaining optional parameters containing additional data that may be useful in the implementation of the filter function:

apply_filters( $tag, $value, $var ... );  

For the example shown in this recipe, a search for apply_filters( 'the_generator' in the WordPress code reveals that it is called within the the_generator template function:

echo apply_filters( 'the_generator', 
                    get_the_generator( $type ), $type );