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