Little Warden can check up to every hour and give you a history of what changed and when so you can take action.
It’s simple to make mistakes though (linking to the http version on a https website, changing the page URL without changing the canonical or just making a mistake in the editing, we’ve seen it all).
With this check enabled Little Warden assumes that the rel=‘canonical’ should appear and be self referential to the URL checked but if it should be elsewhere, you can easily override and you’ll get a notification if that rel=‘canonical’ ever changes.
Little Warden is one of those tools that, once you start using it, you'll wonder how you ever coped without it. Perfect for agencies handling many accounts.