It seems that cron job results are by default, added to /var/log/rsyslog, but I like to have my cron logs seperate, here’s how you do it in Ubuntu 14.04 LTS server.
1. cd /etc/rsyslog.d/
2. nano 50-default.conf
3. Uncomment the line – #cron.* /var/log/cron.log
4. restart the syslog – service rsyslog restart
5. restart cron – service cron restart
Cron results, should now be in /var/log/cron.log