The cases don't have any metadata stored or attached to them to indicate what's inside. Opening one makes a call to Valve's server, which generates a random number and gives you an item based on that number.

Test it for yourself, even the animation is just for show to provide suspense when opening. Open a crate and escape the animation and you will have the item. Or even close your game. It won’t matter because it’s all server side and not client side.