I just released Cacheable Flash 0.1.4. This version includes test helpers so you can easily test your cache messages. It works by allowing you to make assertions on the flash cookie.

Here is a test/unit example:

  require "cacheable_flash/test_helpers"

  class TestController < ActionController::Base
    def index
      flash["notice"] = "In index"
    end
  end

  class ControllerTest < Test::Unit::TestCase
    include CacheableFlash::TestHelpers

    def setup
      @controller = TestController.new
      @request = ActionController::TestRequest.new
      @response = ActionController::TestResponse.new
    end

    def test_cacheable_flash_action
      get :index
      asset_equal "In index", flash_cookie["notice"]
    end
  end

Here is a rspec example:

  require "cacheable_flash/test_helpers"

  class TestController < ActionController::Base
    def index
      flash["notice"] = "In index"
    end
  end

  describe TestController, "#index" do
    include CacheableFlash::TestHelpers

    it "writes to the flash cookie" do
      get :index
      flash_cookie["notice"].should == "In index"
    end
  end

You can install Cacheable Flash by running:

ruby script/plugin install svn://rubyforge.org/var/svn/pivotalrb/cacheable_flash/trunk

See the Cacheable Flash blog post, Show Flash Messages on Cached Pages, and the README for more information.