Does it provide a programmatic notification of being "out of date"?
Does it provide an administrative API?
If the software logs definition/engine updates to the Windows event log you could look for the most recent update, and see when that happened. E.g. with Microsoft Security Essentials in Windows 7, this will tell me the date/time of the most recent definition update (Get-EventLog returns events in order from the event log, so the first match is the most recent):
This is dependent on the Anti-Virus software:
If the software logs definition/engine updates to the Windows event log you could look for the most recent update, and see when that happened. E.g. with Microsoft Security Essentials in Windows 7, this will tell me the date/time of the most recent definition update (Get-EventLog returns events in order from the event log, so the first match is the most recent):
Clearly the details will vary by OS and AV package.
To expand on jdiaz's followup above, the following command will get McAfee's current DAT date and version:
Symantec logs to Windows Event Logs. You can use Get-EventLog cmdlet.