On Redhat systems cron logs into /var/log/cron file. What is the equivalent of that file on Debian systems?
Cron log on debian systems
logs;cron
Under Ubuntu, cron writes logs via rsyslogd to /var/log/syslog. You can redirect messages from cron to another file by uncommenting one line in  /etc/rsyslog.d/50-default.conf. I believe, the same applies to Debian.