How to tell if a website is suffering from a duplicate content penalty across different domains?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  Julius Park: Is there a good way to determine if a website is suffering from a duplicate content penalty across different domains?  ;More specifically, we use the same copy to describe the same service albeit on different websites.  ;Is there a way to find out if Google is penalizing us for this?
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  Jim Munro: I am not an expert, Julius, but I know that there is no such thing as a duplicate content "penalty", the process is more like a filter. Consider that you have an article that is the best result for a query but then you decide to copy the identical article across ten domains. googlebot will not reward the ten duplicate pages with the top ten slots. Apparently, one will be selected, (often it's one of the later-published) , while the rest are filtered out.
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  Julius Park: +Jim Munro ; That makes sense. I'm saying 'penalty' because it might be filtering out the "main" sites in place of the other ones.
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  Raymond Lowe: I think you have to choose which one is the main site to have the good material and basically not reuse identical content.
  
  I've had it happen to me where I loose ranking because an article is used elsewhere.
  
  Surely it is cheaper to get some new original content written, focus on what's different between the sites target market. 
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  Frank Ludriks: You can always use the rel="canonical" tag and point it to the site you want to come up in the SERPS
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  Harry Dance: Hi Julius, if you copy a paragraph from a webpage, put it in quotation marks and search for it in Google. You should see every website which Google classifies as having duplicate content to that page. Do a few of these searches using different pages.
  
  Now check which site, if any, of yours is generally seen at the top of the search engine. Your websites which don't appear at the top, ;Google has classified as duplicate content ;and will probably be affected. When you classify which websites Google classifies as being duplicates, unfortunately you will have to replace their content. While it seems a lot of effort, when you have unique content on each page you will rank for new keywords on different pages
  
  Ps sorry +Frank Ludriks ;the carnonical link won't work like that. It's more for one domain rather than mutliple
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  Julius Park: Thanks +Harry Dance ;
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  Frank Ludriks: +Harry Dance
  Harry you can use the canonical tag across several domains pointing to the original article/page on one domain. Google will then index your preferred domain. http://googlewebmastercentral.blogspot.com.au/2009/12/handling-legitimate-cross-domain.html
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  Harry Dance: Good knowledge, thanks, never heard that before. Surely it's best to have original content on each one though, just so you can come up for a range of keywords across all platforms? 
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  Frank Ludriks: +Harry Dance Yes it's best to have original content but sometimes it's unavoidable to have duplicate content, so it's great we can tell Google which we prefer to have indexed..
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  Harry Dance: Cool, I better keep that knowledge for future press releases
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  Tim Capper: Agree with +Frank Ludriks, canonicolise duplicate content back to the main originator
  
  https://plus.google.com/114669336642325477883/posts/NtjuKvvFNoB
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  Edwin Jonk: From the expert panel in this weeks SEO Questions hangout on air on 00:29:18 into the YouTube video: https://dumbseoquestions.com/q/how_to_tell_if_a_website_is_suffering_from_a_duplicate_content_penalty_across_different_domains +Julius Park
  
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