A default Debian installation will log all messages from the cron daemon in the syslog log file ( it can be found in /var/log/syslog ).

Logging crontab entries to cron.log on Debian

vi /etc/rsyslog.conf

In the file, you will find the following line:

#cron.* -/var/log/cron

Uncomment the line (remove the #) and save the rsyslog..conf file.
Next you will have to restart the rsyslog daemon:

service rsyslog restart

After restarting the rsyslog daemon, crontab log entries will appear in the /var/log/cron.log file.