If I've already modified the .htaccess for 301 redirects and have chosen www at Google's webmaster tools, how do I check to see that my site is completely free from this problem?
open your site with without www - It should not open the site without www and it must redirect to with www version. So, if u open a site without www then it should redirect to site with www It this works properly then there is not any problem of canonicalization