Source

File: wp-includes/post.php

function is_local_attachment( $url ) {
	if ( strpos( $url, home_url() ) === false ) {
		return false;
	}
	if ( strpos( $url, home_url( '/?attachment_id=' ) ) !== false ) {
		return true;
	}

	$id = url_to_postid( $url );
	if ( $id ) {
		$post = get_post( $id );
		if ( 'attachment' === $post->post_type ) {
			return true;
		}
	}
	return false;
}