According to their website:
So from my understanding of preemtion laws it would be technically legal to carry their (feel free to correct me). Me and my family went on Saturday night (I was unarmed) and they are doing security checks on all backpacks, purses, etc. I did not see any signs saying "no weapons" but they are checking for them. I may have missed seeing any signs and didn't have time to chat with the security person other than asking what they were looking for, I had my hands full with my 3 kids (all 4 and under). At the Canyon County fair there was a huge sign at the entrance with rules and #1 was "NO WEAPONS". Caldwell PD was also out in force at the 2C fair. The 2C fairgrounds are run either by the city or county so the legallity of panning weapons may be in question. I did not notice any LEOs at the State Fair except for a few Ada Sherriff vehicles out front, all the security that I saw was private.The Western Idaho Fair, dba Expo Idaho, is a totally self-supporting entity of Ada County, ID
Good luck to you if you try to OC at the fair, it will probably be an uphill battle.