The skeleton helper testcase class generated by Rails 3 inherits from ActionView::TestCase. Simple helpers can be tested by comparing their return value with a golden string. For more complex helpers, use render :text to fill the view state with the helper return value, then use assert_select. assert_tag doesn't work.
Helpers that produce a single tag, or a string, are probably best tested simply by comparing their output with an expected value. Helpers can be called directly from the test class, as inheriting from ActionView::TestCase will set up an appropriate test environment, which includes a mock controller if necessary.
The code listing below demonstrates this.
To me, canonical example of a complex helper is one that includes a call to form_tag or form_for. These methods produce a div inside the form tag, and the contents of the div has been very brittle during Rails 3 development, mostly due to the snowman debate. A test shouldn't depend on the div contents, if all that matters is that there's a form in the helper's output.