Here is a really good firefox addon that shows you the validation status of EVERY page you load, you don't have to do anything other than look at it, ideal for during development and afterwards by clicking through every page to do a quick check. I often find a silly XHTML error can cause other problems, that if you didn't know about you could try to fix another way, and potentially compound your problems by only masking the symptoms of the bug.
https://addons.mozilla.org/en-US/firefox/addon/249