Duplicate content exists when there is more than one version of a page indexed by the search engines. Where there are multiple versions of a page indexed, it’s difficult for search engines to decide what page to show for a relevant search query.
A duplicate content of a website or a blog can be easily identified or judged by a tool named Copyscape. It is a SEO tool that helps to check which content is copied from which source and how much percent is copied. It is the best tool till now. Many other free tools are also there which can be used to check the duplicacy of your content. Don't spam guys as someone is there to punish you.
Identifying duplicate content is so easy because of free tools. Just Google it to know and I will suggest a website smallseotool for plagiarism, just copy and paste entire content of a web page will show you whether it is a duplicate content or not. So, use this site to know about the duplicate content.