I hope u checked point #11
http://wiki.postgresql.org/wiki/Streaming_Replication#How_to_Use- 11. You can calculate the replication lag by comparing the current WAL write location on the primary with the last WAL location received/replayed by the standby. They can be retrieved using pg_current_xlog_location on the primary and the pg_last_xlog_receive_location/pg_last_xlog_replay_location on the standby, respectively.
$ psql -c "SELECT pg_current_xlog_location()" -h192.168.0.10 (primary host)pg_current_xlog_location
--------------------------0/2000000
(1 row)
$ psql -c "select pg_last_xlog_receive_location()" -h192.168.0.20 (standby host)pg_last_xlog_receive_location
-------------------------------0/2000000
(1 row)
$ psql -c "select pg_last_xlog_replay_location()" -h192.168.0.20 (standby host)pg_last_xlog_replay_location
------------------------------0/2000000
(1 row)
Regds
Rajesh Kumar Mallah.