Thank you for your response and sorry for answering so late. Didn't have a chance to check it.
But I cannot figure out how I can measure read vs write activity of my postgresql database with the help of "check_postgres"? I haven't seen anything related to it in "check_postgres --help" output. Moreover it seems I can get all the data that check_postgres provides simply by querying postgresql system tables like pg_stat_database, pg_stat_user_tables, pg_stat_activity and so on.
Let me explain.
When I used MySQL I was able to measure read vs write activity by counting the number of selects vs inserts/updates/deletes etc. But I cannot find anything similar in PostgreSQL.