Enumerate Cron Jobs

cat /etc/crontab                    # system crontab
ls /var/spool/cron                  # per-user cron files
crontab -u root -l                  # root's cron jobs
crontab -u USERNAME -l              # a user's cron jobs

Every user’s crontab at once:

for user in $(cut -d: -f1 /etc/passwd); do crontab -u $user -l; done