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Yesterday, In a Google Hangout on Google+ with John Mueller, John said at the 24:20 mark that using the info command on Google will show you which domain Google holds to be the canonical version in their index.
For example, if you search in Google for [info:searchengineroundtable.com], where searchengineroundtable.com is a domain I have 301 redirected to seroundtable.com. Google is showing seroundtable.com as the main domain.
Actually, it is showing the https://www.seroundtable.com/ as the main domain, which is true. Google has indexed https://www.seroundtable.com/ as the canonical version for all the versions including HTTP, non www, all the domains redirecting to it and so on.
This can be useful to see if Google picked up on specific redirect changes of yours.
Forum discussion at Google+.
