The good news is that it is very easy to find these errors (although fixing them is not always so straight forward).
Firefox Developer Tools
On the HTML tab which gives us a view of all the HTML code on the current page. You can click on each of the different elements and the CSS applied to these sections are automatically updated in the right hand pane, the beauty is you can edit the declarations on the fly and see your changes in real time.
Firebug Console Errors
Errors that need fixing are shown in red. The good news is that the console errors will usually show you the exact file and the exact line as well as the exact error so if you are comfortable in fixing this yourself you can go in and do it yourself, if not you have a very clear and concise error to provide to a web developer or online community like Stack Overflow to look for help.