At the moment xmir does not work with proprietary video drivers. And, as some of us are finding out, whether we get xmir running or not also depends on the graphic adapters we have. There are a couple of threads about this in this section.
I have no problem getting Ubuntu running on xmir. I have a Nvidia GT220 and I am using Nouveau open source drivers. Follow the instructions here
Here are some commands to check if xmir is active
ps aux | grep unity-system-compositor
I cannot provide examples of what you should see as at the moment I am not on my saucy+mir installation. I did post examples in the thread asking about seeing improvements in xmir.
grep -i xmir /var/log/Xorg.0.log
Two weeks ago a sure sign that xmir was running was seeing two cursors but the bug has been fixed. You may see a little lag on window movements and if you move the insertion point through a line of text and then stop the insertion point it will jump ahead one character. This bug may not be noticeable at first but over time it will become noticeable.
Others among us have seen other more pronounced issues.