If an alert has been sent out, you will see it in the app as a pop-up and within the Event Feed at the top of the Home screen. You can also find it in your Alerts module, which archives all the alerts that have been sent out.
Keep in mind that you will need internet or 3G/4G connectivity to receive alerts.
Planners can also opt to send an accompanying email with their alerts. These go to the inbox associated with the email in your attendee profile.
If you are using an iOS or Android device and the planner as opted to make the app available in the app store, you can download it to start receiving alerts with push notifications. These notifications will appear alongside the other push notifications from your other apps, and will also appear as red counter annotation next to the event app icon on your device's home screen.