Monitoring file-based log shipping can be performed in a number of ways. You can look at the current files on both the master and standby, as follows:
ps -ef | grep archiver on master
postgres: archiver process last was 000000010000000000000040
ps -ef | grep startup on standby
postgres: startup process waiting for 000000010000000000000041
This allows you to see the replication delay in terms of the number of WAL files by which the standby is behind the master. Prior to PostgreSQL 9.0, it was difficult to measure the replication delay as a time interval with any accuracy, and some hackish methods were needed. Those aren't presented here. The latest ways of monitoring replication are covered in more detail in the Monitoring replication recipe.