Check the Johnston County system during the event. If there is a large police presence, they might assign them a tactical channel or perhaps one of the interoperability or county ops TGs that are listed in the database. It is very easy to find traffic on these already well-documented systems since you do not have to do much detective work. Just scan the system and listen for any traffic related to the event of interest. If you don't hear anything, then either there is not a large police presence at the event, they were assigned an encrypted TG, or something along those lines.
It is straightforward to scan for event-related traffic on the trunked system yourself.