On master:
select * from pg_stat_replication;
On replica (streaming replication in my case):
select * from pg_stat_wal_receiver;
On your master, pg_stat_replication provides data about ongoing replication:
select client_addr, state, sent_location, write_location,
flush_location, replay_location from pg_stat_replication;
On postgresql v10:
select client_addr, state, sent_lsn, write_lsn,
flush_lsn, replay_lsn from pg_stat_replication;
On server
postgres=# select usename,application_name,client_addr,backend_start,state,sync_state from pg_stat_replication ;
On client
postgres=# select pg_is_in_recovery();
postgres=# select pg_last_xlog_receive_location();
postgres=# SELECT CASE WHEN pg_last_xlog_receive_location() = pg_last_xlog_replay_location()
THEN 0
ELSE EXTRACT (EPOCH FROM now() - pg_last_xact_replay_timestamp())
END AS log_delay;
postgres=# select pg_last_xlog_replay_location();