The problem here is that your server can’t request a page from itself. We should trap this in a better way, but it hardly ever happens.
You’ll have to test it manually:
Make sure the timestamp is enabled on the Debug settings page, then load a page, view the source and the timestamp will be at the bottom of the page. Now, reload the page and the timestamp should be the same. You can also enable the debug log for your own IP and it will show you the page being served from cache.