To test visual design (page layout, color, graphics, and so on), we typically use tools that test an actual working site with production graphics (see Multi-click testing, below) or tools that test a screenshot of a given page (often mocked up in something like Photoshop).
One-click testing
If we want to find out if a particular placement of a control (or its size or shape or color) is effective or not, we can use the one-click tools described in Testing navigation.
Multi-click testing
If we're interested in testing more complex interactions (up to and including actual use of the working site), then we can use the "full-on" usability testing tools described in Testing overall usability.
Next: Testing content