How do I know how much of my plan quotas I have used already?

Head to the management console. You can view usage information and trends, as numbers, charts and as percentage of your plan limits. See the number of images, number of derived images, used storage and consumed bandwidth. Numbers are updated daily.

To help monitor your bandwidth/requests and see where usage is stemming from, for a more in-depth view you can navigate to the Reports page. From there, you'll be able to set a defined period of your choice and identify spikes in usage, see helpful metrics including top requests, images/videos, countries, and much more.

For more information on how does Cloudinary calculate your plan quotas, see:
http://support.cloudinary.com/hc/en-us/articles/203125631-How-does-Cloudinary-count-my-plan-s-quotas-and-what-does-every-quota-mean-