List All CRON Jobs for All Users

You would have to run this as root (hence the sudo):

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

will loop over each user name listing out their crontab. The crontabs are owned by the respective users so you won't be able to see another user's crontab w/o being them or root.