You can view cPanel access, activity, logins and errors for users and resellers on your cPanel server in the following logs:

/usr/local/cpanel/logs/access_log – All activity on cPanel, WHM, and Webmail interfaces
/usr/local/cpanel/logs/login_log – Logins to cPanel, WHM and Webmail
/usr/local/cpanel/logs/error_log – Errors that occur in cPanel

Please note, root access is required to view these log files.