Check here: Unity Mobile Notifications for Android. As long as you can attach some Intent data to your notification, you can always get this data inside Activity. Then if you get null from getIntent() you know you haven’t get there from notification.

From the docs you linked:

If a user taps the notification to open the app, you can get the
notification and retrieve any data assigned to it like in the example
below.

var notificationIntentData = AndroidNotificationCenter.GetLastNotificationIntent();
if (notificationIntentData != null)
{
    var id = notificationIntentData.Id;
    var channel = notificationIntentData.Channel;
    var notification = notificationIntentData.Notification;
}

I’m use Unity Mobile Notification system for my game, but I have a little problem. How can I check if user eneters application via clicking notification?