By default the logging for the cron demon is not active in Debian Linux. To activate it, please open the file /etc/rsyslog.conf via

vi /etc/rsyslog.conf

and uncomment the line

# cron.*                          /var/log/cron.log

After that you need to restart rsyslog via

/etc/init.d/rsyslog restart

and you will find the cron logs in /var/log/cron.log