Since Catalyst 8.4, the crossfire installation problems seemed to have been fixed. (for me anyways) What I did for a quick test to see if crossfire was working properly, was to run firefly forest from 3dMark06. If it's not working properly you will notice almost immediately with the benchmark with stuttering and low drops in fps. If things are running smooth, you have a winner! If things go bad, deffinitely have a look at Dr. Spankenstien's "Making Crossfire Work and Optimization" thread
here. Here are basic instructions on uninstalling video drivers.
Just to make sure.