I will answer them in sequence:
Although it is quite difficult to check the authenticity of a content submitted because with so many Article Snipper (Text Rewriting) tools which rotates English sentences and replaces the phrases with synonyms, its quite tough to differentiate it but quite easy if you thoroughly read the content with open eyes.
- The entire web is indexed and crawled by search spiders every fraction of a second or even faster than than milli seconds so that means, you must not trust any tool but search engines themselves to check against the credibility of a content.
- My best advise is to use Google and Bing Web Search for checking if the content is already indexed or not
- Copy and paste paragraphs that you doubt, which sounds quite general. Never copy the headlines or starting or ending paragraph because spammers often change that and give them a natural touch.
Google always keeps a cache/archives of content from expired domains so you can still search them
Explain this part, could not get you clearly. If 2 are not copied content but one is then simply throw that 1 in trash!
If you realized later that the content is a violated one and already exists on several web pages or atleast one then you may simply delete that article immediately. Google will have already put you in the suspicious list as soon as it crawled that article on your site.
Let me know if I have explained it sincerely