Starting from PostgreSQL 9.4, it is also necessary and useful to check for leftover replication slots. The core issue is that if somebody forgets to drop a replication slot, XLOG can accumulate, just as it does in the case of broken archive commands. Therefore, it is essential to keep an eye on useless replication slots (physical as well as logical).

Here is how you can retrieve a list of existing replication slots:

test=# \d pg_replication_slots
View "pg_catalog.pg_replication_slots"
    Column    |  Type   | Modifiers
 --------------+---------+-----------
  slot_name    | name    |
  plugin       | name    |
  slot_type    | text    |
  datoid       | oid     |
  database     | name    |
  active       | boolean |
  xmin         | xid     |
  catalog_xmin | xid     |
  restart_lsn  | pg_lsn  |

To read data from the view, you can simply use TABLE pg_replication_slots.

The main problem is that an administrator has to know about those replication slots that are...