Most browsers have built in Web Developer Tools or extensions you can download.
In Chrome (and to a certain extent Safari) you just press F12 and they pop up as a separate window and allow you to view the HTML and when you click on a particular element it shows you the CSS affecting that element.
You can also right-click and ‘Inspect Element’
Firefox works pretty much the same but has a great extension called Firebug which adds even more.
IE (also F12) has the same but, IMO, it’s not as well developed as the more modern browsers.