I've released a gem that lets you assert in rspec whether a given action or fragment has been cached. It works with memcached only.
Example:
lambda { get :index }.should be_cached(controller, :action => 'index')
Available at: http://rubygems.org/gems/caching_matcher
Github: http://github.com/mightyverse/caching_matcher
Please check it out.
Best,
Wolf