I understand that it is a good idea to add a canonical link to each web page.
Read this Webmaster Tools Help from Google (it will explain how and where to put it):
A canonical link element is an HTML element that helps webmasters prevent duplicate content issues
If you have not applied canonical tag for your website then first you have to check it in google that how many pages has been crawled from www and non-www. If your webpages has been crawled from non-www then you to apply it for www to non-www and visa versa.