Hi Guillermo. You would use a tool like Firebug in Firefox or Google Developer Tools in Chrome. Right-click on your page on, for example, the menu. Then select “Inspect element” from the pop-up menu. This will open a window at the bottom of your display with the related line of HTML code highlighted. For each element you can see the assigned ID (id=””) and Class (class=””) names. When used in CSS, element IDs are prefixed with a pound sign (#); classes are prefixed with a dot (.).