You can view the information from either on SQL query by gateway information. Or through ART (Administrative Reporting tool). If you dont have it installed on the call manager, then click on Application and install plugin and then install the software. The default username/password is admin/admin.
Those are the only 2 ways I know to find out which gateway the call is passing through. The only differnce in both of these are the sql is not user friendly but is in real time, ART is 24hr. delayed but the reports are provided in readable format for the end user.
This URL should help you:
http://www.cisco.com/warp/public/788/AVVID/artlogic_5300.html