Here's a better link for the first method mentioned above, it's also available here on RG. A couple of other methods are also published using either LC-MS or HPLC-UV, so you can just make your pick based on which equipment you prefer or have available. What you need to be sure of is that the method does not allow uncontrolled hydrolysis of CMS into colistin during sample work-up as is the case in microbiological assay methods and at least one HPLC based method.

http://www.sciencedirect.com/science/article/pii/S073170850800664X