Crontab problem with backup

I was trying to backup a database called "old_db".The following lines
inserted into either root crontab file or postgres crontab file failed
either to produce any output or created an empty file caled
"old_db_hr9.tgz" in the /backups/hour/ directory..

0 9 * * 1-5 postgres pg_dump --format=t old_db >
/backups/hour/old_db_hr9.tgz

0 18 * * 5 postgres pg_dump --format=t old_db >
/backups/week/old_db_friday.tgz


What logs do I need to check to find the problem? What might be a way to
test the script?