Caching can speed up your site and extensively lower the load on your server. If you are using a Content Delivery network (CDN), Varnish or another caching mechanism, the fastest method is to check the http headers. Although headers may be modified for reasons like security, they usually expose if your caching system is working or not.
So you have your caching mechanism on your website all set up and working. Great! Once the cache has been built, your pages will be delivered to your visitors as fast as possible. But have you also thought about warming your cache?