I have never had to treat for mites but based on what I understand, you can usually see the mites. You can usually see black specks all over the snake and some will move, or you can see it in the bedding or water dish. Another clear indicator is that your snake sits in it's water bowl a lot because it is trying to drown the mites. To treat it, a lot of people recommend Provent-a-mite by pro products. It is an aerosol that you spray on the bedding and you let it dry before putting your snake back in.
Here is a video that shows how to treat a lot of snake illnesses. Based on my research, these are commonly used treatments. Just copy the url into your browser.
http://youtu.be/pJCnPfuqaBM