The Whole Server:
You can run this command, which will list all the existing cronjobs for all the users, including root, on the server:
for user in `whmapi1 list_users | egrep -i ".*-\ .*" | awk '{print $2}'`;do printf "Username >>>>>> %s \n" $user && crontab -l -u $user;done
The output looks like this:
Username >>>>>> cptest
no crontab for cptest
Username >>>>>> root
@reboot /usr/local/cpanel/bin/onboot_handler
30 1 * * * /usr/local/cpanel/whostmgr/docroot/cgi/cpaddons_report.pl --notify
31 3 * * * (/usr/local/cpanel/scripts/fix-cpanel-perl; /usr/local/cpanel/scripts/upcp --cron > /dev/null)
0 1 * * * /usr/local/cpanel/scripts/cpbackup
0 2 * * * /usr/local/cpanel/bin/backup
35 * * * * /usr/bin/test -x /usr/local/cpanel/bin/tail-check && /usr/local/cpanel/bin/tail-check
0 6 * * * /usr/local/cpanel/scripts/exim_tidydb > /dev/null 2>&1
30 5 * * * /usr/local/cpanel/scripts/optimize_eximstats > /dev/null 2>&1
5,20,35,50 * * * * /usr/local/cpanel/scripts/eximstats_spam_check 2>&1
45 */4 * * * /usr/bin/test -x /usr/local/cpanel/scripts/update_mailman_cache && /usr/local/cpanel/scripts/update_mailman_cache
30 */4 * * * /usr/bin/test -x /usr/local/cpanel/scripts/update_db_cache && /usr/local/cpanel/scripts/update_db_cache
25 */2 * * * /usr/local/cpanel/bin/mysqluserstore >/dev/null 2>&1
15 */2 * * * /usr/local/cpanel/bin/dbindex >/dev/null 2>&1
*/5 * * * * /usr/local/cpanel/scripts/dcpumon-wrapper >/dev/null 2>&1
0 */2 * * * /usr/local/cpanel/scripts/shrink_modsec_ip_database -x 2>&1
1,16,31,46 * * * * /usr/local/cpanel/whostmgr/bin/dnsqueue > /dev/null 2>&1
5 4 * * * /usr/local/cpanel/3rdparty/bin/freshclam --quiet --no-warnings
22 22 * * 7 /usr/local/cpanel/scripts/send_api_notifications > /dev/null 2>&1
09,39 * * * * /usr/local/cpanel/scripts/clean_user_php_sessions > /dev/null 2>&1
For A Specific User:
If you already know the user and you only want to see the list of cronjobs associated with that specific account/user then you can use this command: (Replace $USERNAME with the username of the account)
cpapi2 --user=$USERNAME Cron listcron