Canonical Tag Checker
What Would Cause You To Need A Canonical Tag Checker?
Google sees a duplicate content issue when you publish the same exact content on more than one web page on your website or blog. Naturally, each page will have its own unique URL. Therefore, they will be considered as different pages that have the same content. Your website users may sometimes ignore the fact that they are seeing the same article on more than one location. However. the problem starts when people stop seeing your web page in Google SERPs which will definitely decrease the number of people reading your great article that you may have accidentally published across multiple pages. This would be because Google would have stopped including your article page in its index until you've fixed the duplicate content issue.
The simple solution is to determine which page you first published your article and delete the others. But what if each duplicate page has already been linked to by people from their own blogs? You can't delete them now, can you? You are now left with two solutions: 301 redirect the duplicate pages to your chosen original page or use canonical tags. If you've chosen to use canonical tags, make sure that in all those canonical tags, the canonical URL you use is the page URL of the article page which you want Google to consider as the original page.
And then, use this free Canonical Tag Checker to see whether your canonical tags are showing up in all the pages where you've added them. Of course, you can just press CTRL + U on your PC keyboard to view each duplicate content page's html source code. But if you want to do your checking much faster, it would be wise to use an online tool that specifically checks only the presence of canonical tags in any web page on your site, a friend's blog, or a client's ecommerce store where the occurence of duplicate content issues can be overwhelming. You'll be happy you're using a canonical tag checking solution that helps lessen computer eye strain.