Normally, when connected to the Internet, the software will inform you when a newer version of the software is available by displaying the latest version available near the bottom of the Options menu.
Example:
However, the version shown there will not always match the latest version. That is because when we release a new version of the software, we don't want to inform all customers simultaneously if the application includes any new issues we missed during QA testing. After enough feedback has been collected, we will update the latest version indicator.
The newest version of the software can always be found on the Saleae download page.