How can I determine whether a student has written an excellent paper themselves, or hired someone else to do it? In class, over the course of a semester, I tend to build up a mental model of individual students' communication abilities. For example, one student might seem more or less eloquent than another when they speak. One might make more cogent points than another.

Occasionally, I will receive an essay which appears exceptionally eloquent and cogent, from students who seemed distinctly average. Of course, I make the cursory checks for plagiarism (searching certain unlikely phrases, etc.), but if nothing comes up, I sometimes still remain unconvinced that the work is the student's own.

Some sites offer to sell the services of trained academics and graduate students for menial essay writing, and often the questionable assignments look more like the quick and under-edited work of a master than the polished work of a novice. Of course, if the essays are paid for, and original, it's naturally very hard to prove it.

Are there any techniques I could use to determine whether an essay that I suspect was written by someone else, but not published elsewhere, is a student's own work? Are there any steps I can take to ensure I don't receive questionable essays in the first place?
Are there any techniques I could use to determine whether an essay that I suspect was written by someone else, but not published elsewhere, is a student's own work? 


Ask the student to come to your office hours or a private meeting. 

Then, say: "I was really impressed by your recent paper, it was an excellent piece of work! Let's discuss it some more."

Student who did the work will be able to discuss said work intelligently.

Student who didn't do the work is unlikely to be able to carry on an intelligent conversation about said work. 

(This still isn't proof of any kind, and you can't accuse student of cheating without some evidence - but if you're lucky, student will at least be spooked enough to never do it again.)


  Are there any steps I can take to ensure I don't receive questionable essays in the first place?


Assign essays that are a few steps beyond, but still closely tied to, what's been discussed in class, such that somebody who wasn't in the class will not be able to produce the kind of essay you're expecting (the effectiveness of this depends very much on the subject matter).