How to check that cache is working on current page?
The easiest way is probably to inspect the HTTP response headers. 

For example, with your browser DOM inspector tool ( e.g Chrome's "Network" tab )

Cache Miss

Here's some sample response headers from drupal.org showing a cache miss. In this case, using Varnish however core Drupal cache also sets similar headers.



Cache Hit

Here's one showing a standard Drupal cache hit: