Depending on the state there is no way to acquire a certification prior to some down and dirty on the job training. I'm a dispatcher in Oregon and the states public safety and standards division requires minimum and maximum hours to complete certain stages in training prior to allowing a certification. It even seems silly because local colleges offer classes is emergency communications but as far as I know they don't count for much, just a little dip into what it will be like prior to being hired.
I'm not 100% sure if there is any way to do a sample criti-call test like at home, but that sure would be nice.
I also agree with mfdbenji's point. I find it a little irritating when people question what or why I tell them to do something. When you are dealing with a live call or dispatch scenario and your coach tells you to do something, it's for a good reason, just do it. You can ask questions after. I also think it's very important not to be too sensitive. I've seen people wash out because they couldn't handle harsh criticism, but when you are dealing with potentially life or death situations it calls for a more abrupt intervention or action from your coach. He also made a good point with what we call "styles." Everyone has a different style of doing something, and over the course of your training at most agencies you will have several coaches. You learn each of their styles and once certified it leaves the foundation for you to create your own style.
Good luck in getting hired tho, but find outlets and ways to tell yourself that you did a good job. Rarely will the public ever thank you for what you do.