I've never been to a faire where they asked me at the gate to let them see what was in my flask (when I bring one), unlike for example, a concert.
I don't think I've ever seen 'rules' actually forbidding outside refreshments/food at a faire, but since most outdoor shows (racetracks, concerts, amusement parks, etc.) don't allow it, I would just assume the faire has the same rules. At KC I did see a few folks with open sodas and beers get stopped at the gate and told to throw them out before they could go in. The venue sold alcohol so I could sort of see why they wouldn't want you bringing in a cooler, but these were single cans and bottles, open and being consumed.
I guess a good rule of thumb would be if you pay to get in, you can't bring food or drink past the gate. I suppose you can get a stamp and make trips out to your car if you want to save money on refreshments.