Synchronize with Active Directory and run an existing report showing agent version by computer
Query WMI for Win32_Service for name AeXNSClient
Look in registry for existence/value of HKLM\Software\Altiris\Altiris Agent\Version
Since I don't know what you want to do with the data, I'm not sure which option is best. If you want to deploy the agent next, I'd recommend option 1 or 3, if you just want a simply check, option 2 seems fine.