how can I test rails cache feature

Well, first, you shouldn't really be testing the framework. Rails' caching tests ostensibly cover that for you. That said, see this answer for a little helper you can use. Your tests would then look something like:

describe Tag do
  describe "::all_cached" do
    around {|ex| with_caching { ex.run } }
    before { Rails.cache.clear }

    context "given that the cache is unpopulated" do
      it "does a database lookup" do
        Tag.should_receive(:order).once.and_return(["tag"])
        Tag.all_cached.should == ["tag"]
      end
    end

    context "given that the cache is populated" do
      let!(:first_hit) { Tag.all_cached }

      it "does a cache lookup" do
        before do
          Tag.should_not_receive(:order)
          Tag.all_cached.should == first_hit
        end
      end
    end
  end
end

This doesn't actually check the caching mechanism - just that the #fetch block isn't invoked. It's brittle and tied to the implementation of the fetch block, so beware that as it will become maintenance debt.