I am not clear on the laws for Cashman Field. It is not clear to me as to what they use to enforce their policy. However, I have in the past tried to open carry at the gun shows.
Each time I was directed to a table where there was Metro officers setting. The past norm was they would look at your firearm if it was loaded they would ask you to leave and store it locked up in your car, or empty it and bring it back which seams asinine sending people out into the parking lot to handle their loaded firearms. Once back with the empty firearm they would verify it is empty, put a zip tie through the action, and hand it back.
For some reason I cannot remember for sure if they did an ID, permit, or blue card check at that time. I do not believe they did. I would like to think I would have left before showing them anything. Other posters will correct me if I am wrong as to if they did ask. Of course, if one is going to purchase a firearm be ready to show a ID or two.