Well, assumed that the user is either listed in /etc/cron.allow or not listed in /etc/cron.deny, i.e. the user is allowed to create his own cron table, there should be a file for every user that is allowed to and has a crontab in /var/spool/cron/crontabs/.

Besides editing (crontab -e) or listing (crontab -l) his own crontab entries, root can also specify the name of the user whose crontab is to be tweaked with

crontab -u user -e

See also crontab manpage