I was wondering which cache plugin you are using in your WP demos, in case you are actually using one.
Many cache plugins add an HTML comment at the close of the page which would give you an answer, ex:
<!-- created by hyper cache: xxxxxxxx 11-04-03 08:55:37 -->
I use massive amounts of caching on my sites. For WordPress, I use WP Total Cache for static file caching and memcached for queries. Here’s a neat tool: http://codex.wordpress.org/Transients_API. The WordPress transients api lets you store arbitrary strings, arrays, or objects in the database with an expiration. If you use WP Total Cache with memcached, it puts these in memory. This is useful if you are spending time computing things like popular posts or pulling feeds from APIs on the server side.
Lately I’ve been experimenting with CDN mirroring using Amazon Cloudfront, but there are some issues I need to work out with that.
I use w3 total cache for everything, but mainly because it allows me to use a CDN