How to checked all sites content for uniqueness

I'd go with copyscape anytime - You don't need their paid/premium service, if your content is already published - just get the link to your content, and paste it into their search, and you'll be able to search for free. But if your content isn't published on a website, then you'll need to get their paid service instead.
 
Copyscape is the industry standard. It's what we use for our client work. I'm hearing good things about Quetext too; yet to try that out though.
 
Copyscape is industry standard. Grammarly is also equipped with a similar feature, and it's free to use.
 
if you want to to check pages in one website, you can use the free tool screamingfrog (limite to 500 urls for the free version)
 
Copyscape is pretty reliable for such use-case. As mentioned here it is used and preferred by most people.
 
Copyscape.com enables you to feed it urls for large scale plagiarism checking
 
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