Option #1:
By default cron jobs log to /var/log/syslog
You can see them by: grep CRON /var/log/syslog

Option #2:
Open file /etc/rsyslog.d/50-default.conf
Find the line that starts with #cron.* /var/log/cron.log
Uncomment that line and save the file
restart rsyslog: sudo service rsyslog restart
Now the cron logs appear in /var/log/cron.log

Option #3:
in crontab at the end of the task add >> /tmp/cron.log 2>&1