Look at the various components under the LANDesk tree node of the inventory and query against the version number of one of those. I've tended to use the inventory scanner in the past.
LANDesk Silver ESP
The One-Stop Shop for LANDesk Enhancements
What version did you have running before you upgraded to SP3? Querying off of the inventory scanner version is what I used to use until SP2 when LANDesk started releasing the montly component patches and file versions on those patches where actually in SP3 as well. If you are upgrading from an earlier version before SP2, then this might not be an issue. One thing I started doing was renaming my agents according to versions. For example, now that I upgraded to SP3 I added v9.3.0 to the end of my agent name. Whenever I need to change the agent due to updates or setting changes, I just change the last number. This is really the only sure way I came up with for tracking agents and making sure they are up to date.
Mike, for future reference I highly recommend that when you make changes to your agents (significant) that you add an attribute to the agent config name and keep a spreadsheet with the details.
For example, I would name an agent like this:
Win PC Agent v1.2
Win PC Agent 2012-03-20
This makes it easy to see when / what agent has been deployed, it is easy to query