As root, how can I list the crontabs for all users?

Well depends on the script but easily you can find your crontab as root with

crontab -l -u <user>

Or you can find crontab from spool where is located file for all users

cat /var/spool/cron/crontabs/<user>

To show all users' crontabs with the username printed at the beginning of each line:

cd /var/spool/cron/crontabs/ && grep . *

One liner which lists all users and prints cron for every user:

for user in $(getent passwd | cut -f1 -d: ); do echo $user; crontab -u $user -l; done

This solution:

Doesn't require knowing a system specific crontab path (e.g. /var/spool/cron/crontabs/ vs /var/spool/cron/
Won't list "orphan" crons, i.e. crons for users that don't exist anymore (and thus are not executed effectively)

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