As Cliff has posted there are several ways of making it obvious that rounds are dummies. It shouldn't be a problem to tell.
NCOWS Bylaws also require that NO LIVE ROUNDS be anywhere on the person at any re-enactment event. While the Convention may not technically be a re-enactmnent in the traditional sense, the rule on live ammo certainly applies.
We all want and need safety to be first and foremost, but with proper procedures in place, this issue shouldn't present a problem.