you can check plagiarism easily just search on Google 'plagiarism checker' and you will get some website to check it. The result will shows that how many copies are available on internet.
I tried some plagiarism checkers on student essays in Spanish and found that I get better results by checking suspicious sentences with Google. The checkers probably work better on papers in English.
Just today I read a sentence that seemed suspicious, so I simply typed it into Google. The first webstie to pop-up contained the infraction, along with several others throughout the student's paper. I've used Google several times with successful results.
There are quite a few "originality checkers" but I also like to use my own impression. You should know your students well enough to recognise if that writing is really theirs. Such a review goes not only to word choice but also sentence and paragraph structure and construction. If you have doubts take a sample sentence or phrase and throw it through your favourite search engine. I get better result with that than with Turnitin or any of the others.
My major complaint about turnitin is not so much turnitin but the academics who don't understand how to use turnitin.
If the university has an anonymous marking policy of which many universities do, it may be difficult to link a student to the strength of their work. This a response to Rob Gutt
My own experience is that we mark our students' assignments [Africa]. But be that as it may, if you are dealing with first year students and the paper reads like it was written by a PhD candidate then the chances are that it is not original work. Then you need to find the source.
My experience is that many staff view the Turnitin result and penalise the student accordingly. That is just plain wrong, and common sense needs to be reintroduced.