The cancan:ability generator should create a test/spec file to encourage testing of the Ability class. It could contain something like this.
describe "Ability" do
describe "as guest" do
@ability = Ability.new(nil)
it "can only create a user" do
# Define what a guest can and cannot do
# @ability.should be_able_to(:create, :users)
# @ability.should_not be_able_to(:update, :users)