Is there any SQL command that I can run on the slave database to check if the replication is still working or not? I know that you can check the postgresql.conf but I want to do it directly on the SQL panel.
Database: Postgresql 9.1 on Linux
You can query pg_stat_replication (since 9.1+) on the primary server, it will return one row for each secondary connected.
If you were on 9.2+ you could even use pg_xlog_location_diff to get the lag in bytes of data sent to the standby:
SELECT application_name, client_addr, pg_size_pretty(pg_xlog_location_diff(pg_current_xlog_location(), sent_location)) AS sent_lag
FROM pg_stat_replication;
You can use write_location and replay_location similarly.
On 9.1 you'll have to do this math by yourself if you want or create your own pg_xlog_location_diff.
OBS: pg_stat_replication will only show standby connected through streaming replication, log shipping cannot be verified this way.