How to check the replication delay in PostgreSQL?

I would like to measure time between insert data into master-table and slave-table using streaming replication in PostgreSQL 9.3. For this I create table test_time with 2 fields id(serial), t(text). After that added a trigger:

cur_time:=to_char(current_timestamp, 'HH12:MI:SS:MS:US'); update test_time set t=cur_time where id=new.id;

But the time is the same in both tables. How can I measure delay time