Criti Call
It is my understanding that they use what is called Criti Call. Basically it is a software program that was created to test people to see if they can be trained as a dispatcher. It test your typing ability, multi tasking skills and ability to prioritize calls.
MSP just started doing this last year. When they started this state police dispatch (control) thing 8-9 years ago they were looking for cops, firefighters and or EMT's not actual dispatchers (this comes from the mouth of someone that worked for many years as a MSP dispatcher and was hired at the beginning). They are now smarting up and looking for people that want to make dispatching a career since it is a profession in of its self.
I had to take it when I interviewed with the City of Northampton. I passed so I would have to say it is not all that hard.
This will give you some incite. It is a website where you can see a demo (most have power point).
http://www.biddle.com/criticall/demo.htm
P.S. Let me know which Control (dispatch center) you are applying for and I can give you some insight (maybe).