Source

File: wp-includes/widgets.php

function is_dynamic_sidebar() {
	global $wp_registered_widgets, $wp_registered_sidebars;

	$sidebars_widgets = get_option( 'sidebars_widgets' );

	foreach ( (array) $wp_registered_sidebars as $index => $sidebar ) {
		if ( ! empty( $sidebars_widgets[ $index ] ) ) {
			foreach ( (array) $sidebars_widgets[ $index ] as $widget ) {
				if ( array_key_exists( $widget, $wp_registered_widgets ) ) {
					return true;
				}
			}
		}
	}

	return false;
}