We're so glad you asked!  There are a few ways to be notified (via our newsletter or social accounts), however the source of your OS X (Mac) updates will always be from the App Store application on your dock or Applications folder.

As far as how you get notified and what actions are taken regarding your Mac App Store applications?  These settings can be found in System Preferences > App Store

Here you can adjust your settings on handling updates and the action you'd like them to take

If you're not sure what you have or what the latest is, you can always launch App Store and click on Purchases at any time. If you're not already logged in, it will prompt you to log in with your Apple ID and password to show you your previous purchases.  These are also the steps you would take to reinstall a previously purchased app.