Hi Ed,
You'll likely want to find the element where "Too Short" will be listed and then do a "get_text' on that element and verify it contains the text "Too Short". I do something like this when I'm working with text verifications like that (in Python):
element = driver.find_element_by_css_selector("input#password_strength")
element_text = element.get_text
self.assertTrue(element == "Too Short", "%s does not match. Expecting "Too Short". Got %s") % (element, element_text)
Hope that helps,
-Doug