When building a new skin it can be useful to make 'Display Debug Comment' checked in ThemeBuilder > (select skin) > Options tab
This will display extra information when viewing html source or using a tool such as Firebug in Firefox.
See the Debug section on this page for more information - Skin Editor - Options
You can also add a query string to the end of the page address to force Confluence to use any skin when ThemeBuilder is the Theme for that space.
Example: Add 'skin=root' to see how that space will display in the default ThemeBuilder skin.
(if this is the first parameter in a page address, use a '?' to preface - if there are other parameters use '&')
This is a great tool for comparing small changes against the default ThemeBuilder skin, and is also good for troubleshooting if you are not sure if something in your work has broken the current skin.