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

I have a script being run automatically that I can't find in the crontab for the expected users, so I'd like to search all users' crontabs for it.

Essentially I want to run a crontab -l 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 . *