It’s fairly simple, you can use google uncle to find out if an article has been copied and also find the source of it.
But often who copies is intelligent enough to change some words, vocabulary or turn around the content is some way so that google is not able to search the exact copy. But I don’t see that as a problem, if you are smart, you must use your smartness.
Google search isn't the only tool to use. For a more accurate search you would need to use multiple tools to detect and verify plagiarism. Moreover, these tools are just an aid to speed up the search. For me, personally, I would need to check the article manually to validate fraud before I downvoted.