doing_filter() │ WP 3.9.0

add_action( 'wp_head', function(){

	doing_filter( 'wp_head' ); //> true

	var_dump( doing_filter('wp_head') );

	die;
} );

As a Result: When the wp_head action is triggered, WP will be interrupted and the bool(true) will be displayed.

The example shows how you can dynamically change the email header depending on which filter is triggered by the same function.

add_filter( 'wp_mail_from',      'filter_system_from_mail' );
add_filter( 'wp_mail_from_name', 'filter_system_from_mail' );

function filter_system_from_mail(){

	if( doing_filter('wp_mail_from') )
		$opt_name = 'admin_email';
	else
		$opt_name = 'blogname';

	return get_option( $opt_name );
}

doing_filter() is able to check the "chain" hook being executed.

Suppose a function is attached to hook foo_filter and it calls hook bar_filter. Inside the function attached to the hook bar_filter you can check if the hook foo_filter is currently running.

add_filter( 'foo_filter', 'foo_filter_function' );
add_filter( 'bar_filter', 'bar_filter_function' );

echo apply_filters( 'foo_filter', 'Hello' ); //> Hello world!

function foo_filter_function( $text ){

	return apply_filters( 'bar_filter', $text );
}

function bar_filter_function( $text ){

	// Let's check the execution of two hooks at once: 
	// - the current one (bar_filter) to which the function is attached
	// - and the parent one (foo_filter) to which the function that 
	// caused the current 'bar_filter' hook is attached.
	// I.e. at the moment 2 hooks are executed at once: 
	// 'foo_filter' and 'bar_filter' and the function.

	// doing_filter() can check any of the currently running hooks
	if( doing_filter('foo_filter') && doing_filter('bar_filter') ){
		return "$text world!";
	}

	return 'Noooop!';
}

As you can see from the code, in the function that is attached to the hook bar_filter both conditions are triggered: doing_filter('foo_filter') and doing_filter('bar_filter'). Which means that the two hooks are currently running at once.