Enterprise Alert relies on several notification channels to deliver the messages. In some cases the mobile app is not used as the primary channel. Because of this Managers are not notified if users are not logged in to the app. Thankfully, emails, SMS and voice calls are all still delivered regardless if users have logged into the mobile app, or even have it installed on their phone.
However, if you are using team schedules you are able to see if there is a gap in the schedule and this could be an indication that alerts may be missed. On the Who's on Call page a scheduled team with no one on duty will show as gray instead of blue.
Teams where the scheduled user has alerts turned off will show orange.
There is an additional option in the Team Scheduler that will notify you if there are gaps in the schedule. This will be delivered via email to the Team Manager.