I'm gonna assume you are talking about latest versions of an app? You know by going into the App Store. If there is a new version for you to download, you will have a badge showing you that there is a pending update to an app.
There is also a setting in your settings app to enable automatic updates, so if there is an update to the app, it will automatically download and install.
If there are no update badges in the App Store, then you most likely have the latest version of the app