use function to detect if slug exists in WP databas

if_slug_exisits.php

<?php

/*
* Use the function  if ( the_slug_exists( '404' ) ) {} to detect if is in database 
*/
function the_slug_exists($post_name) 
{
    global $wpdb;
    if($wpdb->get_row("SELECT post_name FROM wp_posts WHERE post_name = '" . $post_name . "'", 'ARRAY_A')) {
        return true;
    } else {
        return false;
    }
}