Symantec Endpoint Protection
There is no common library in use on operating systems to tell which definitions are in use. However, for each anti virus application you could find out if there's an API to call. Symantec Endpoint Protection goes without, but it stores activities in a Syslog compliant log file (syslog.log). See if that log file contains information about updates and definitions.
Clam AntiVirus
Anti virus integration with .net application
McAfee VirusScan
A powershell script by Ying Li to check against a list of servers for McAfee virus definitions installed.
http://www.myitforum.com/articles/40/view.asp?id=10522