You would have to run this as root, but:
for user in $(cut -f1 -d: /etc/passwd); do 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.
if you want to know which user a crontab belongs to, use
for user in $(cut -f1 -d: /etc/passwd); do echo $user; crontab -u $user -l; done