Successful Agent Install in the Event Viewer
To get a little more detail, take a look in the Application section of the event viewer. If you sort by "source", you should see a lot of entries from "MsiInstaller". From there you can see a history of all installs - both successful and not.
Here is an example of a successful install:
Event Type: Information
Event Source: MsiInstaller
Event Category: None
Event ID: 11707
Time: 7:05:24 PM
Product: AgentInstall -- Installation operation completed successfully.
For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.