I have had to show a racing license at all the USAC road and track races in which I participated and I have volunteered for registration and if someone showed up that was licensed but did not have the license with him/her, we got online and checked the USAC website using the racers name on the other form of ID. So no one did not get to race due to lack of a license with them.
As a side note, I use the USAC phone app and have my license available electronically. Of course, one could forget ones phone.