is_active_sidebar( string|int $index )

function is_active_sidebar( $index ) {
	$index = ( is_int($index) ) ? "sidebar-$index" : sanitize_title($index);
	$sidebars_widgets = wp_get_sidebars_widgets();
	$is_active_sidebar = ! empty( $sidebars_widgets[$index] );

	/**
	 * Filters whether a dynamic sidebar is considered "active".
	 *
	 * @since WP-3.9.0
	 *
	 * @param bool       $is_active_sidebar Whether or not the sidebar should be considered "active".
	 *                                      In other words, whether the sidebar contains any widgets.
	 * @param int|string $index             Index, name, or ID of the dynamic sidebar.
	 */
	return apply_filters( 'is_active_sidebar', $is_active_sidebar, $index );
}