While the tools provided by W3C are likely to be the most accurate, I would also suggest testing your site with the online tools provided by 3rd parties, just to be thorough. If there is any discrepancy between the results, then it might pay to take a closer look at your HTML code.
If all of this content is flying over your head, then I would suggest sourcing a website developer/coder to take a look at your site.
In an effort to judge site authority and quality, its likely that Google will include W3C validity in their algorithm in the future; so now would be the time to get your code up to scratch.
Courtney is an online marketing and communications specialist at ineedhits - a leading search engine marketing firm with over 16 years experience. Courtney has been living and breathing online marketing for over 5 years. She specializes in web and communication marketing, while providing news and opinion to online marketing communities.