You can check any installs by looking at the log files or by checking the System Report.
An organized list of software including dates can be viewed by using the System Report. To access select "About this Mac" from the Apple Menu in the main menu bar. Select "System Report" from the window that opens. Select "Installations" from the Software section. This list can be sorted by Software Name, Version, Source or Install Date.
The install logs are stored in /var/log and can be viewed either in Console (Applications->Utilities) or with your editor of choice in Terminal (Applications->Utilities). Software that uses Apple's installer will log to this location.
Logs can also be viewed from the System Report
For a list of approved updates see KB0011722
If you are curious about an Apple update, you can read more details at http://support.apple.com/downloads/.
Security patch information is available at http://support.apple.com/kb/HT1222.