If you have upgraded to Mountain Lion and found things to be a bit on the flaky side, or simply want to know if there are updates available, you can change the frequency of checks made by the OS.
Run the following from the terminal, specify the interval in days as the last parameter:
sudo defaults write /Library/Preferences/com.apple.SoftwareUpdate ScheduleFrequency 2
To get the current setting, swap read for write and remove the last parameter:
sudo defaults read /Library/Preferences/com.apple.SoftwareUpdate ScheduleFrequency