If soft deleted, you don't need any "keys" to find it. For hard deleted it's not a simple matter to recover. To find data on deleted posts, it's most likely in your moderator logs at admin.php?logs/moderator/. But if they are hard deleted, they aren't going to be "recoverable" other than by restoring a backup or a LOT of work to try to export all the necessary data pertinent to those posts and update the associated tables (why it's not usually recommended to use hard delete unless you KNOW you want those threads/posts gone immediately). Soft delete you can wait a bit since they aren't visible to anyone other than mods/admins, and if after a week or two you don't need them, go back and hard delete them.