Canonical Tag Checker
Check any page's canonical tag: is it present, is there exactly one, is it absolute, and does it point to itself? Catch duplicate-content issues. Free.
Frequently asked
Should every page be self-canonical?
Usually yes — a page should declare itself the canonical unless it's intentionally a duplicate of another URL. A canonical pointing elsewhere tells Google to index that other URL instead.
Why does a relative canonical matter?
The spec allows relative URLs, but absolute canonicals are far less error-prone across protocols, www, and trailing slashes. Always use absolute.