Ideally, your current hosting provider can supply you this information.
If not, and provided you have Google Analytics installed, you can do some back-of-the-napkin math by multiplying your total number of page views per month by your average page weight to figure out roughly how much monthly bandwidth you are using.
You can use a service like GTMetrix to determine your average page weight.
Our plans each have an amount of CDN and Bandwidth allocated by default. The CDN amounts are always higher allotments — about 5x the allotment for Bandwidth — due to the cost-effectiveness of CDN to Bandwidth, and the general ratio of CDN to Bandwidth for a well-functioning site.
Said another way, CDN is much less expensive than Bandwidth, so we try to cache as much of your content as possible.