Is there a service which allows you to check if a paper/article was plagiarized?
At this moment, I use Google to check if some papers are plagiarized or not, but I think that are other smarter methods
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There are many software options available (see Wikipedia's page on plagiarism detection), some free, and some paid.
To my knowledge, one of the most commonly used commercial options available is Turnitin; there is also Turnitin's student version, WriteCheck. Another commercial option is PlagScan. These options can be somewhat expensive though; PlagScan, for example, charges by the word: $13 gets you 50,000 words of plagiarism checking.
There are several free services, but they should be used with caution and their limitations should be understood. Some, for example, simply do what you've been doing: they take the sentences and search in Google. This "direct plagiarism" is much easier for the software to look for. Others, like the Viper plagiarism-scanner, do a pretty good job in my experience (from a long time ago), but still only match direct matches, and won't catch things where a word or two have been changed here or there. I've since stopped using them because they have a somewhat sketchy data retention and policy: whatever you upload, they add your work to their "study site" (or essay mill or whatever you want to call it). (To Viper's credit, they are transparent about how they use your data!)
If you are checking against a known local database of documents (which it doesn't sound like you're doing, but I thought I'd mention it), you can check out WCopyfind. It might not be the most intuitive, but it is promising and free. You might use this software, for example, if you have a database of papers students have submitted in the same course offered in different years, to check whether a student in the current batch of students is plagiarizing from an earlier student or from someone else in their class.
When reading a plagiarized research paper turned in by a student, the signs of plagiarism may not always be obvious. However, there are some things that professors can pay special attention to when suspecting plagiarism.
Observe citations throughout the paper: This will help determine whether or not the required citation style was used. For example, if the citations in the paper conform to Turabian style when the MLA style was required for the paper, you may be looking at a plagiarized paper.
Observe the writing style or styles used in the paper: Sometimes, you may notice that some of the writing used in a paper does not "sound like" something the student would write . The unusual words or wording may include "jargon or advanced vocabulary". Other papers may include a writing style that is below average.
Look for signs that the paper is outdated: Sometimes, professors can find signs that the paper being read may be outdated. If all of the sources listed in references or works cited are several years old, then the paper may be an example of plagiarism .
Observe the font used in the paper: A paper may contain more than one "font style or size." Different fonts throughout a paper would indicate that a student may have copied and pasted various portions of Web pages together into a word processor.