Source

File: wp-includes/theme.php

function is_random_header_image( $type = 'any' ) {
	$header_image_mod = get_theme_mod( 'header_image', get_theme_support( 'custom-header', 'default-image' ) );

	if ( 'any' === $type ) {
		if ( 'random-default-image' === $header_image_mod
			|| 'random-uploaded-image' === $header_image_mod
			|| ( '' !== get_random_header_image() && empty( $header_image_mod ) )
		) {
			return true;
		}
	} else {
		if ( "random-$type-image" === $header_image_mod ) {
			return true;
		} elseif ( 'default' === $type && empty( $header_image_mod ) && '' !== get_random_header_image() ) {
			return true;
		}
	}

	return false;
}