One of the most useful features of modern browsers is that they all come with an extensive set of tools for inspecting how your CSS styles are interacting with your webpage's HTML. My preferred browser for debugging web sites is Google Chrome - here's a tutorial on how to use Chrome's developer tools:
The section on "Inspecting the DOM and Styles" should be most helpful for this chapter.
While I like Chrome's dev tools best, it's definitely a personal preference. Here are links to developer tools documentation for other major browsers: