Since WordPress is open source, another way to find information about hooks is to search directly within its code. For every action hook that accepts user functions, you will see a call to the do_action function to execute all the registered items. As can be seen, the function takes two or more arguments, with the second one(s) being optional:

do_action ( 'tag', [$arg] ); 

For the example shown in this recipe, a search for do_action( 'wp_head' ) reveals that it is the only function that is called when a theme makes a call to the wp_head() function in its header file:

do_action( 'wp_head' );