If the developer has approved them for being available on Mac, they are (of course) updated in app-store, like other apps from app store. If you want to trigger a check for updates press cmd+r for refresh on the update page.
I assumed it was via the updates in the App Store... but for an app I installed that was updated a while ago on the iPad... the updates screen showed nothing.
However cmd+r fixed it and immediately the updated version appeared for download.
Not sure why I had to force that using cmd-r but it certainly wasn't going to do it by itself!