There are two ways you can check who is currently logged into WHM or cPanel.
1) Log in to the server via SSH and run ps aux | grep whostmgrd to see who's currently active in WHM, and ps aux | grep cpaneld to see who's currently active in cPanel. If the users have been idle for some time, this method may not work.
2) This method is less than ideal, but if you know the IP address then you can tail -f /usr/local/cpanel/logs/access_log | grep ip.address to see what they are doing, again only if they are active within WHM or cPanel will this work.