How do I find out who needs to update their mobile app?

If you navigate to Users page, there’s a list of accounts, that can be filtered with ‘Outdated’ control at the top of the table.

App versions are automatically tracked against the latest one available in App Store, so you always know who needs to update. You can select all those users and send them a push message with a reminder if needed.

Additionally, User details page displays exact version number, that is highlighted in red color if outdated.