The best way to collision check in WP

I don’t think there is a built in mechanism for searching the meta table for a key/value pair other than a full blown post search with WP_Query. I’d consider that overkill. Just query the table:

$unique = $wpdb->get_var(
  "SELECT meta_id 
  FROM {$wpdb->postmeta} 
  WHERE meta_key = 'unique_code' 
  AND meta_value="{$random}" 
  LIMIT 1"
);

I can’t help but think that more detail in the question might point to a better way to proceed