Detect Canonicalization Problems

Detecting canonicalization issues is easier (thank God) than defining it.
I usually apply one or all of the following 3 techniques:

1: Detecting Canonicalization Issues Using Google Webmaster Tools

This section assumes you’ve already got a Google Webmaster Tools account, and that your site’s verified. If you don’t, or it isn’t, go do it. I don’t care if you think Google is spying on you – they’re doing that anyway. You may as well get the benefit of the toolset.
Here’s how you check for canonicalization issues:
In Webmaster Tools, click ‘Diagnostics’, ‘HTML Suggestions’. Then check for pages with duplicate title tags. If you see a list like this:

…Google’s detecting duplicate title tags on your site. Assuming you’ve used unique title tags on your site, canonicalization is the most likely cause of these duplicates.
For each duplicate title tag, click the ‘+’ sign. If you see two URLs that are really similar, I’ll bet my hat you’ve got a canonicalization issue:

Regardless, click each page URL and view the pages. If the content matches, you’re in canonicalization purgatory:

Record the URLs for every instance of canonical chaos (sorry, couldn’t resist). Google Webmaster Tools makes this easy: You can just click the handy ‘Download this table’ link and get a CSV file.

2: Detecting Canonical Issues Using A Link Checker

You can also use a link checking tool and list page title tags. Again, look for duplicates and check for canonical confusion.

3: Using Search Results

If you used the same title tag on every page of your site, then title tag reviews won’t help. Here’s what to do:

Go to a page of your site.
Copy one phrase that you think is likely unique. So “Click here for more information” isn’t a good candidate. “Curmudgeon’s Web Copywriting Site Clinic on Tuesday” is a good one.
Go to the search engine of your choice, and search for site:www.yoursite.com “your phrase”.
If you see something like this, you may have a problem:

This is a very poor method for detecting canonicalization issues, because it’ll detect every single instance of duplication, throughout your site, regardless of the cause. It’s also hit-and-miss, because search engines may actually drop some duplicates.
But, if you click ‘repeat the search’, you’ll get a list of duplicate pages. Then you can sift through the list and check for problems.

Another option: Getting fancy

You can also build your own site crawler, if you’re a hardcore geek. We did that at Portent years ago, and use it to automatically detect canonical problems.
If you don’t live and breath PERL, PHP or Python, though, I don’t recommend it. You’ll end up like me.