The only way I think you can check that is
Login to your cpanel ( admin ) > Setup > Users and Group > Yes, I have users - Next > It would tell you when you need an upgrade for me it says
To add more than 10 users you'll need a Google Apps for Business or Education account,..
OR (if this works) https://www.google.com/a/cpanel/DOMAIN.COM/CPanelHome#SetupW...
change DOMAIN.COM to your domain