Unlike other Linux distributions, usually you will not find /var/log/cron.log file in ubuntu so it becomes difficult to find cron logs on a Ubuntu server where you have configured cron jobs and you want to check crontab status with logs.
However, if you want to see the file cron.log appear in Ubuntu server, follow this easy steps. I tried this setup with Ubuntu 10.04 LTS Server.
The following steps will enable the cron log to allow for better debugging details.
Edit /etc/rsyslog.d/50-default.conf and uncomment the line with cron.*
Run /etc/init.d/rsyslogd restart
Run /etc/init.d/cron restart
Hope this will help.