The HTML Validator will enable you to check the validity of a list of URLs according to W3C standards.
Making sure that all of your pages are valid is a great asset. A validated website means that your pages are cross-browser compatible and also easier to be parsed by search engines. Moreover, validated pages permit to people with disabilities to access the content of your pages.
Even if one cannot affirm that a non-validated page will directly affect the SEO results, a non validated page might directly affect the user's experience and therefore the page's popularity, the number of bookmarks, the number of visits etc...
It is therefore highly recommended to validate all your website’s pages by using this XHTML Validator.