If you use the internal alarm clock then an overlay window should appear on the top of the screen.
This window is on top of any other app and can't be missed. It stays until you:
- press one of the buttons (snooze or dismiss)
- press the volume down button two times, then the alarm is also dismissed.
If there is no windows popping up I assume you might have changed the allowed notifiations for the clock/alarm app (you can check in settings --> apps
