On your CCM webadmin page:

Got to aplication->Cisco Callmanager Serviceability->Tools->Serviceability Report Archive

OR

Use RTMT. Open RTMT->Performance->YOur CCM-> Cisco H323 (or MGCP, depends on what you are using)-> Double Click Call Active-> select Gateways you have-> you will now have a graph showing the number of all B Channels used, right click on it select-> Alert / Treshold-> Enable Alert (Severity: Warning-> Click Next: on "trigger alert when the following condition meet" select a Value ie Over 25 (which means when 25 B channels=Calls are simultanueously used you will get an email alert of it). Do this for all Gateways and all Callmanager processing Calls.

HTH

Karzazi