To validate the fraud0-event in Google Analytics (GA4), you need to trigger the custom event with the GA4-tag first and check if the event is properly sent to your property.
Go to your Google Analytics Account
Navigate to your property and go to Reports -> Real Time in the left-hand navigation bar
You should now be able to see the F0-event in the Event count by Event name tile
The real-time report only shows data, that has been collected in the past 30 minutes.